I'm reading this a few years late, but thanks for the ball point pen idea! Saved the day.
My tool dealer has them per ten, hundreds or thousands.
I believe that this particular one is one of the larger, perhaps 1mm diameter or a bit more, for this Pentax lens.
In the future, when removing that aperture ring, do so in a clear food storage bag so that nothing goes flying.
Nota Bene for all: for literally more than a decade I have been saving tiny screws, ball bearings, tiny springs, etc, that I have garnered though junk bodies and lenses that I am not able to fix. I save these precious tiny parts in small, plastic coin rolls that have a screw on cap. This is probably the most valuable source of parts that I have and I recommend that, before you through anything out, you scavange these tiny parts. They WILL save your day. - David Lyga
If you haven't found some drop me a PM with your address.
I have 100 new 1 mm sitting unused.
Easy to zip-lock 5 or 10 in an envelope and send them your way.
Thanks for that tip David. I always lay everything out on a nice clean rag or towel before taking anything like that apart, but it's of scant help if something flies off, as has happened more than once. I actually found one of those little bearings once when it flew off into the shag carpet. What's the odds of me doing that again? Nada. From now on, in the bag she goes first hand.
Coarse rags or towels also help to prevent ball bearings from rolling away.
I forgot: is the 2/50 pentax the M42 or the PK mount? The PK mount one has a tiny, tiny ball bearing that GOES FLYING when you take off that aperture ring. The older, M42 lens has a LARGE ball bearing, maybe 2 ml even, and is easier to manage.
If my walls had memory and vocal chords they would be able to recant the swearing that I have done over the years about these utter frustrations. - David Lyga