Hi, I have the original complete rear light unit to fit
But I don't know what fits into the hole on top of alloy holder!, there are the two holes for bolts on the bottom of the bracket.
Depending on year, there were 3 types of alloy rear light unit. One type fitted for the 67 Baracuda (Starfire if US model), then one for the 68 model year (long type which covered the rear mudguard support loop) and one fitted for 69 and 70 models (short type). If I'm translating your question correctly, and assuming you mean the 68 model year housing, there should be a stud in that position (BSA part no. 31-0222 which is 1/4" BSW/BSF x 7/8"). HTH
Thankyou for your help