Auto Rikenon 28mm f2.8 m42 Lens Filter Size?

I recently acquired a first-generation Auto Rikenon 28mm f2.8 m42 lens, and looking to get a yellow lens filter for it. I've been trying to find info on this Tokina-made lens in hopes to discover what exact filter size it is. After looking around and comparing, my first guess is that it would be 58mm, but not certain. Would y'all agree?


http://m42lens.com/m42-lens-database/1185-rikoh-auto-rikenon-28mm-f-2-8-16

Thanks!
-Paul

Do you have a lens cap for it that clips into the filter threads?
If so, check to see whether the size is indicated on it.

@MattKing
Negative. No lens cap. Wish I had one though.

Should be pretty easy to figure it out with a simple ruler or more precise a caliper.

@Dali
Yeah, a caliper would be key. I'll figure it out. I just thought that I'd throw this out there to see if anyone a little more familiar with Auto Rikenons than I am.
-Paul