I measured the threads on the front of my Industar 50mm f/3.5 lens (for the purpose of adding a close up lens). I got 34.89mm inside, which means the threads are probably in the range of 35-36mm, oddball.
I want to mount my 37mm close up lens, the thrifty Sony MT-2037S. It's achromatic and quite good. I may duct tape it on for a trial, but will order a couple of step rings in the meantime. More to come.
Followup 2015/1/10: I googled the filter size. Little did I know, there were two versions of this lens with different filter threads. Old ones (pre-1970??) were 33mm, recent ones are 35.5. You have to measure yours to know.
Nix the duct tape. I ordered the 35.5 to 37 step up ring from Forum Sponsor Adorama so I could use all my 37mm attachments. It's here:
Adorama Step-Up Adaptr Ring 35.5mm Lens to 37mm Filter SUR35537
If by chance you have the older lens with 33mm filter ring and need a 33 to 37 step ring, here it is:
Metal Step Up Ring 33mm to 37mm 33 37 Sonia New Adapter | eBay
By the way, the second vendor calls himself "Pentax Shop" on ebay. Great Feedback. Worth checking out if you need accessories.