Hello. I am hoping you can help me. I have been experimenting with adapted manual lenses. I just bought a Sigma XQ 28mm f2.8. It was sold with an M42 mount advertised. However, when it arrives there are some irregularities with the mount when compared to my Helios and Hanimex lenses. I posted photos showing the difference. 1. Is this a M42? It does screw down to a certain point, but the square "pin" and blade (flange? - see the photos), means it will not screw down enough against my M42-NEX adapter to actually depress the infinity-focus pin. I noticed this on another M42 on sale at ebay by the way, so any background information would be helpful to me and appreciated! My adapter has a lip to depress the infinity-focus pin. Is there a type without it? If so, problem solved (?) 2. If not M42 what is it? The lens is in *MINT* shape, so I am loathe to give up on it. Any help would be appreciated!