large threaded hole in rear triangle of rigid frame?

[Invicta]

This may seem a silly question, but can anyone please tell me what the 3/4" diameter threaded holes in the rear triangle of a rigid frame model 18 are for ?. They do not appear in the diagram in my 1949 parts book, but in the AJS parts and year guide by John Ready showing part number 013773, which is the correct frame number for 1949 , they appear to have been added in free hand. I have seen photographs on the internet of other 1949 and later rigid frames that also have these holes with nothing in them.
When I checked my other AJS rigid framed bike, which turns out to be a bit of a hybrid, having a 1950 500 cc engine in a 1954 model 16MS frame. At least that is what the club archive says that frame number A7231 should be. I believe that the S is for a spring frame though mine has a rigid rear end. Could be that a P.O. at some time in the past put a 1954 front frame and forks onto the 1950 rear. I suspect this was done, as it also has the 1954 head light with the two torpedo pilot lights on . Anyway on this bike there are two large nuts, with a 3/4" diameter shaft sticking out of them just over 3/4" long with a 1/2" diameter threaded hole longitudinally through them, one on each side of the frame. These being screwed into what I believe are the same holes as on the 1949 frame. Just what were these fittings for? Does anyone know?

[Pharisee]

For a sidecar chassis mounting.

[Stan Palmer]

Sidecar for sure. But a shot in the dark about those tubes: could a previous owner have been in those games they used to play in post war gymkhanas, the passenger had a Lance or a net as part of the game ? Sorry, probably COVID madness !
Stan

[Invicta]

Thank you for that gents. I did think about sidecar mounting points, but UK sidecars were always mounted on the left. I suppose they put the holes in both sides for export and those who wanted to fit a sidecar on the right side of the bike. I think the gymkhana suggestion may not be too far fetched in relation to the short threaded bars fitted on my other bike. I did take some photographs but don't seem to be able to attach them. Look forward to seeing if anyone else has any other ideas as to what they might have been for. I will remove them when I get round to doing the restoration on it. For now still working on getting the 1949 model 18 back together. The basket is empty and a lot of bits are missing.