Sounds like the lip on the mount has been bent slightly out of shape - it happens on some of the cheap M42-K adapters too. Bending it back and getting it right is nigh-on impossible unfortunately.

You could try a call to Sigma - they must have access to spare parts and they might sell you a mount for a reasonable price, although it's more likely they'll ask for the lens to be sent in to them for repair (which I doubt will be anything like as reasonable!).

Good luck!

Looks like the retaining ring on the rearmost element has become unscrewed - should be simple enough to screw it back on.

Rather than trying to disassemble the lens see if you can do it in situ: Use tweezers to reseat the ring over the threads and use something like a pencil eraser to screw it down.

If you can't get a good enough grip that way then take the back (mount) off for better access - be warned though, small parts may drop out as you prise the mount off (usually 1mm ball bearings that act as the "clicks" when you turn the aperture ring) so make sure you do it over a container (I use a cake tin made from a one-piece pressing so there are no seams and it's deep enough for me to work inside). If they get near your carpet I guarantee you will never see them again!

I'd never seen that before - added now to my bookmarks

I had a couple of 75mm's I was watching that finished today, unfortunately I've been out since early morning and I've only just got back so I've missed them. I had spotted the Nikon but ruled it out due to the (a) price and (b) number of others who seem to be interested in it.

Still have a couple more options up my sleeve though...

I love misspelt items on eBay - less people to bid against me

The apostrophe in the title maybe?

Zenits are actually still in production and those made in the last 10 years or so have a K mount. Hardly ever see them though as they're no longer sold outside Eastern Europe / Russia so any you find over here will almost certainly be M42.

It's now on my watch list in case I don't pick up one of the earlier ones, although the Nikkors seem to fetch more money than I strictly want to pay...

The cameras are generally dire in quality and heavy as bricks but the lenses can be very, very good. Nearly always M42 mount (although very few of the later ones were K mount) so you'll need an adapter but the Helios 44 (58mm f/2) is a stunningly sharp Zeiss Biotar clone and well worth having. These were the kit lenses and almost every Zenit you ever see will have one attached.

Any idea what the lenses you're picking up are?

Excellent stuff! I've lined up aselection of 75mm lenses that finish over the next 2-3 days so I should bag at least one of them! Looking forward to giving it another try now