top of block, between pots 3 & 4. its just above oil filter housing, flat area. probably too rusted to read now but a bit of wire brush actiuon and shining a bright torch at different angles and you might be able to make it out. not that it really matters as all 1.8 bottom ends fitted to MK1 & MK2s are the same (except G60) carb, k-jet and digi. only difference is on carb theres a fuel pump bolted to the front, k-jet has a mounting plate for the wur where the pump would be, and digi just has a plain blanking plate here.
carb wont have an oil cooler and often has only 1 hole in the filter housing, but these unbolt/swap over. oh and coolant pipes might be different ue to oil cooler/metal pipe arrangement etc.