Forget about the side marker, this is a Euro spec Alfa in Europe, its a repeater. The matching bulb is an all glass bulb and it has only one filament. Repeaters gets the live from the front direction indicator and when the GTs were new and the socket were made from metal they got earth thou contact with the metal wing. If the socket is of plastic it needs a wire to earth.If it is a turn signal in place of the side marker, it still has to be a side marker. It appears to need both the side marker wire (yellow) and the turn signal wire (blue and blue/black).
I don't know if there are two bulbs in there, or one that will go dark with the turn signal pulse when running lights are on.
NB! As far as I remember when we in Denmark imported a US spec car we had to disconnect the side markers to get them approved - the front simply was rewired to be a side repeater and the rear we simply removed the bulb. I and others imported some MG Bs in the late '70s when the MG B was only made RHD for the UK market and LHD for the US (we transformed the engines to Euro Specs as well as all the emission control robed 25+ HPs).