Just received lens that -- surprise! -- wasn't quite what I hoped it would be.
The front cell is engraved "Doppel-Anastigmat 1:6,8 f - 13 cm". No maker's name anywhere on it.
It is in a dial-set Compur with C.P.Goerz engraved on the speed dial.
No serial numbers anywhere, just what I think is a German patent (D.R.P.) number on the shutter's face plate.
I've counted reflections, saw four strong, no weak from each cell. So it is a dialyte type. Uncoated, of course. It passes light and forms an image. The focal length is very nearly, if not exactly, 13 cm, so I think it isn't a pair of loose cells someone screwed into a shutter that happened to be lying around. #0 shutter, as near as makes no difference.
Any suggestions about how to tell which Goerz lens it is, and even whether it is a Goerz? I mean, Goerz applied several names to their dialytes in the '20s.