Thank-you for all this interesting information on something that I hadn't known existed until recently. I need to make a slight correction to my last post, in order to be fair to the dealer concerned, which was caused by having a bad memory and not having the actual advert in front of me when writing the post. The advert read that the sight was rare not very rare as I stated and so many apologies for that mistake.
So if I have understood matters correctly a SLR with this type of rear sight will also have a different fore-sight fitted too? Would it be a simple job to replace the original standard sights or would there be modifications that would need to be undone? Thank-you to everyone for all the information.