Vintage shoulder flap. Single chain. When you open the bag the chain loops through the round grommets and hooks onto the sides of the bag with small S-locks. This is a plus if you have heavy items because the chain is connected to the main base portion of the bag and not just the flap. In a regular 2.55 classic flap the chain is connected to the flap only, and so the weight of the bag is all hinged on the clasp, which holds together the top and bottom - chain pulls the bag upwards and weight pulls the bag downwards. If you snap the double flap portion closed, then the snap helps with the weight support. That is also why you see a lot of cracked snap buttons because of the pressure. Most people keep the interior flap open though. With the vintage flap you mention, the chains are connected directly to the base "unit" therefore eliminating strain on the front clasp and flap portion. Of course this is only if you plan on carrying heavy things.
It's a single chain that is not long enough for cross body for most people. The chain length is usually around 18" to 20." A good cross body needs about 22.5 to 24" to be comfortable. I mean drop, the length from the top of the shoulder to the top of the bag. Not the length of the chain. You could try to "8 loop" the chains to do the double strap thing but it would not be long enough to carry on the shoulder anyway. Maybe just a clutch. Which is fine - some people will double up the chain and wear it as a clutch. Worn naturally, it is worn on one shoulder and falls around your waist. You can also shorten the chain by "cinching" it inside.
For the black lamb, interior is burgundy, signaling a vintage classic. 18k gold plated hardware and very durable structure makes it popular among Chanel enthusiasts because the quality they say is so much better. Very well-structured. This model comes in 9 inches, 10 inches and 11 inches. Production year early to late 90's. Comes in both Caviar, Lamb, and patent, and was made in Black, Navy, Red, White, Beige, Pink, Brown, Olive.