Looks like it could be a Nikon F mount but that lens would also have been made for Canon FD, M42, Pentax K etc

I'm okay when they're in front of me but not so good at being able to tell from pics, sorry

that lens mount is a pretty distinctive one, with its wide flange and the very short flange. Though I don't recognize it, someone will.

And third-party lenses often have a hard-to-notice compatibility code, so carefully check all the markings.

Hi,

I am really struggling to work out what this lens fitment is. I have put a EOS - NEX adapter to sho the size difference. The internal diameter is much smaller than the EOS. The lens is a Sigma 600mm reflex.

TIA

Well that's not going to work...