This lens may well be a fitted with what was called the Adaptall 1 mount. I have never actually used one. The Adaptall 2 mount had 2 small chromed tabs inn the adapter which were squeezed in and then the Mount would be able to be removed in a straight lift off. It is an age since I used or even owned one of these so I may be wrong. If you do manage to remove it make sure that you note where the mount came from on the lens because they can be tricky to fit back on.

Why give up looking for a Zuiko 21mm? as 21mm's go they are a cracking bit of kit. The F3.5 version is not uncommon and not too expensive. In UK I currently know of 2 for sale around the 200 mark. They will 100% certainly be better than the Tamron.