@8x57 thanks for the reply, they are off a older 90's Steyr, Im guessing they are 16mm dovetails or there about. I was thinking they might be optilock, looks like the rear one is adjustable as well? Just wondered if any knew, before I go overing £80's worth of rings to find out I was wrong!!!
Weren't Tikka rails 15mm?
If you are only looking to replace only the rings and not the mounts why not unscrew the rings and measure the bottom portion of the ring. I'm sure that someone on the site will confirm the diameter of the ring on the bottom of their Optilocks for you.