The Finnbear is an L-61R, and if it is a Whitworth thread form, it will be a 55 deg V instead of a standard 60 deg V. I've read and been told that many gunsmiths use a std 60 deg threading tool to cut tenon threads for Mauser actions with the 55 deg Whitworth receiver thread.
A thread pitch gage will tell you what the pitch is, but you may not be able to tell the difference between a 55 deg and a 60 deg form with it. Last year I had a chance to record the dimensions of a L-61R take-off barrel. The barrel thread O.D. was 1.050" diameter X 16 tpi, and the tenon was .677" long, shoulder to breech face. I never thought to check if it was a 55 deg thread form, but if it was, it wasn't obvious, because the pitch gage (std 60 deg form), fitted too well.
I believe the previous replies had you covered, but since I had been able to measure one and record the dimensions, I decided to reply also.
I really like the Sako actions, but JE is right. I have a .30 x .378 take-off barrel from a Sako M-995, and it's definitely a metric thread. I think the pitch on the thread is between a 16 tpi and an 18 tpi. Nothing I had matched up. I don't have a metric pitch gage to verify it.
If you decide to rebarrel it, the L-61R is a very fine action.
Good hunting, Tom