Since long I have a Summitar, but quite recently acquired many original filters for it, for very decent prices (about Euro 20 each). Indeed most filters come with pressed glass (glass is fixed) and some filters with a detachable ring. The glass itself is 39mm so you can switch glass with the glass of 39mm filters (of course also the IR/UV 39mm for your summicron).
As others I like the rendering of the Summitar a lot, mostly for b&w pictures which provides a really retro picture. I bought my Summitar (1940 edition) about 20 years ago. One should really be careful buying one these days, because as with the summars and summicrons, many have a lot of (very small) scratches (aka cleaning marks), in my view it is not worth buying one with cleaning marks. For summitars also a distinction is made between post-WWII and lenses before: this has to do with the difference of the diaphragm blades (see other threads on this issue).