Steve—this is a collapsible Summicron 50/2, made in 1956 (judging from the serial number)
Jja—thanks for looking! The front and rear elements of mine are pristine, no cleaning marks, and it appears that it's been cleaned before, so no haze inside either. There are two 'droplet' looking marks inside, that look like coating defects, but those don't seem to affect pictures.
Krosya—I didn't keep detailed notes, but with the exception of the last photo (that was using f/2), the rest were made between f/2.8 and f/5.6.
Tomasis—I'm not sure about the 'glow' and three-dimensionality of the pictures, but I like the results from this lens.
Certainly, it is less contrasty than my Summicron-M (the current version with built-in hood), but most of my lenses stopped down to this degree, as Krosya says, are all very good.