Build Sticker - Sort no. question

Hi All,

Between the excellent information contained on these forum pages and a number of other sites, including a couple of German ones, I have been able to completely decipher the entire build sticker (Ecotag) found in the boot and typically stuck on the front page of the service book by supplying dealers, with one important exception:

At the start of the Sort No. is a four digit code, which in the case of my car (and many other 1983 GTi's and Campaigns) is '1212'

Can anyone definitively tell me what these four digits signify and how you know this to be the case please?

Many thanks,

Jim