I have the same thing stamp on my lock too and I purchased my birkin a few months ago from H directly. I was curious about the extra num on the lock since the lock was one of the ways for ppl to tell if it's authentic. I asked my SA and was told that it couldn't be a date, it might just be something new that H is doing with their locks. The craftsman in nyc told me the same thing too.
For all you know, Hermes does not wish to reveal the logic for the number engraving, to stall the scammers from hitting the production line and speed away with making fake Hermes locks for the rest of time!
Hi djsmom & cxyvr, I have the same 09/06 on my lock, I just saw a bag on ebay with the 01/07 . Great way to date the bag with the lock if that is their intention!