The serial number on the bottom bracket reads: H3M5 4603 94087. The 94 indicates the production year, which is supported by the stamp "1993" on the fork. XXXX XXXX YYXXX is the composition of the serial number from 1994 onwards, a final confirmation of the production year of my GT.
My final quest is to find out the further logic of the serial number. The first 4 numbers look different, are on the left. I assume that therefore they have been stamped before the other numbers, maybe by the supplier. My guess is that the first 4 numbers are to do with the frame and the model. H3M5 therefore indicating tubing specifics, maybe also the size?