Originally posted by puderse Can't be that rare! I've got a black one #5,2XX,XXX. Works perfectly but I can't/don't know how to test the auto flash thingy. It's there in my 'M42 at the library' shot.
You need a Honeywell Strobonar flash. There were two or three models, all of which are available on eBay for quite reasonable prices.
I believe that all the Strobonars whose model numbers end in 2 will work. For example, the Auto Strobonar 772 or 882. Those two are the "potato masher" configuration, that mounts to a bracket on the side of the camera.
I never understood quite why the IIa was never sold outside the US as an Asahi Pentax. The Strobonars were made by Rollei in Germany. I would have thought that Rollei would have produced them under their own brand.