Here is some info from an eBay reseller:
For many collectors of Chanel costume jewelry, the time period when the jewelry was first manufactored and sold is as much an issue as its authenticity. Here's my personal guide to using the Chanel signature not only as a mark of authentication but also as an indication of when it was orginally sold. The guide is a result of several months of research and examining several hundred different pieces of Chanel costume jewelry.
Disclaimer: Please keep in mind while reading this guide that there are exceptions to every rule; that is, there may be authentic Chanel costume jewelry with a signature other than those I describe below. My research is also ongoing, thus the content of this guide will be revised to reflect any additional information I acquire and improvements I think will otherwise add to the guide. If you are interested in contributing additional information that will help me to ensure the accuracy and quality of this guide, please send me a message via eBay so I can incorporate it. Last but not least, I would like to emphasize that this is a personal guide. I do not work for Chanel, nor is this guide by any means official in the sense that it is endorsed by Chanel, Inc. or its parent company at this time.
1920s through 1930s: Chanel costume jewelry produced and sold prior to 1939 and before Chanel first closed her boutique was designed specifically to accompany the various suits, day dresses and evening dresses she sold at that time, and is unsigned. The lack of a signature makes it hard to authenticate, but rarely will you find very early pieces of Chanel costume jewelry available on eBay anyway. Most of it is in the hands of serious collectors.
1940s: There are some costume jewelry pieces, notably enamelled brooches, that were originally sold in 1941 with name Chanel stamped on each piece in script. This jewelry is not associated with the house of Chanel but rather was produced by a French costume jewelry company called the Chanel Novelty Company. Chanel sued to have the company change its name, and it shortly thereafter became the Reinad Novelty Company. The jewelry has value in and of itself for a number of reasons, but it shouldn't be mistaken for Chanel costume jewelry. There is no authentic Chanel costume jewelry with the Chanel signature in script.
1950s through 1960s: Gabriel re-opened her boutique on Rue Cambon in 1954. As previously, much of her costume jewelry was designed and produced to accompany her clothing. Along with the chains she used on the hems of her jackets to weight them properly, necklaces, earrings and brooches (most notably the camelia flower) were included and sold with each outfit. In the 50s and early 60s, many of Chanel's costume jewelry pieces were designed by the young and talented Robert Goossens. The majority of pieces from this time period are marked simply as CHANEL, usually stamped directly on the piece itself.
The same signature, possibly with some minor variations, continued on most if not all pieces until Gabriel's death in 1971.
Circa mid 70s to mid-80s: Copyright and registration trademarks began to appear on Chanel costume jewelry around the time Alain Westheimer took control of Chanel in 1974. The signature plate typical of the mid-70s to early 80s includes the signature CHANEL CC MADE IN FRANCE stamped on a round plate or sometimes on the piece itself. The CHANEL is accompanied on either side by copyright and registration trademark.
Beginning 1980 and until 1985, the date on most piece substituted for the phrase MADE IN FRANCE. Variations I've seen on this plate include: an oval rather than a round plate, and copyright mark next to the CC logo as opposed to one side of the word CHANEL.
Circa 1986 through 1989: Karl Lagerfeld was appointed the couterier for Chanel in 1983. Shortly thereafter, he hired Victoire de Castellane as head designer of Chanel costume jewelry, where she remained head designer until 1998 . Pieces designed by de Castellane circa 1986 to 1989 are identifiable by their oval signature plate. At the top of the plate is the word CHANEL, accompanied on each side by copyright and trademark. At the bottom of the plate is the signature MADE IN FRANCE. In the middle of the plate is the CC logo with the number 2 on the left side and an additional single-digit number on the right.
The numbers on both sides of the logo identify the season for which the jewelry was produced. 2 CC 3, for example, identifies the piece as produced for Season 23 and 2 CC 5 identifies it as produced for Season 25. Most of these plates are soldered or otherwise attached directly to the piece, with a few pieces (e.g., bracelets) marked by a hanging oval tag with the same signature found on other pieces from this period of time. To the best of my knowledge, the first of these signatures begins with Season 23 and ends with Season 29.
Circa 1990 to 1992: the signature plate is oval with CHANEL and its copyright and trademark registration marks at the top of the plate and MADE IN FRANCE at the bottom. The middle of the plate is marked only with the CC logo.
1993 to present: Beginning 1993 the year and season (Spring or Fall) began to appear on either side of the logo in the middle of the plate (e.g., 93 CC P). A "P" signifies a Spring season. and an "A" signifies Fall. Again, most of the plates are soldered or otherwise attached directly to the piece, with a few pieces such as charm-style bracelets marked by a hanging oval tag with the same signature found on other pieces at that time.
The oval plate with CHANEL, the Chanel copyright and trademark, and the year and season accompanying the logo remains the signature to present day with some minor variations. Increasingly, the mark at the bottom of the plate on many contemporary pieces reads MADE IN ITALY rather than MADE IN FRANCE. More of the signatures tend to be stamped or engraved on the pieces themselves, rather than on a plate attached to the piece; I've seen them stamped directly on the lobster clasp of necklaces or bracelets, for example, or on the back of a brooch. Another change for Chanel is its addition of earrings for pierced ears, and rings. Earrings for pierced ears, which are much smaller than the clip earrings previously standard for Chanel, may have no signature because there's no available place on the piece to really be able to put one. On rings, the signature is usually very small and lightly engraved or stamped somewhere on the inside or outside of the band. A "C" indicates that a piece was produced for the year's cruise collection.
According to a fashion feature in the May 30th 2010 edition of the New York Times, Chanel is the one brand of vintage costume jewelry that "never seems to tarnish" in value.
