Photo is a bit blurred but I am guessing it is a square headed coach bolt, ie it has a smaller square below the head to stop it turning when fitted in a square hole or in a slot and has a whitworth thread. Looking around this link might be useful https://www.myersblacksmiths.co.uk/#/rivets-bolts/4565523902
The Amal carb in the illustration I copied differs slightly in that the float chamber is under the carb and held by two screws. Your carb has the float chamber on the side and I think you will find the original top would have screwed into the float chamber. The thread may have got damaged and hence the replacement top.
Seeing the photo from busman it does look as if you have a non standard top to the float chamber. I am sure a photo giving a better view of the rubber bung and surrounding area will confirm what it is.
The pool of knowledge within the club is vast and unfortunately some will be lost over time unless it is passed on, either by word of mouth or written down, (puts hand up with guilty look on face).