Rigid and DR are optically the same lens, Raid, they were announced together and manufactured in parallel. Optical blocks within a certain measured focal length tolerance were used for the DR, to guarantee close focus accuracy, while others were used for the rigid. Also, mechanically, except for the google transmission, they are built the same. The rigid handles a little differently, since it has an infinity lock, where the DR doesn't.
Both are the same first Mandler design, and also the first computer optimized Leitz lens. For M-mount, they were announced in parallel in 1956, the rigid as 11118 / SOOIC-MS and the DR as 11318 / SOOIC-MN (later SOMNI). The screw mount version of the rigid (for the IIIG) was catalogued in 1961 and called SOSTA.
Nowadays, when in similar condition and for M mount, the rigid is worth more since it fits Bessas and digital Leicas, where the DR has to be irreversibly modified (breaking the close focus ability). A clean rigid can easily go over US 1k these days. On the other hand, KEH is currently selling an EX DR without googles for US $849. In my experience, clean copies of either are very rare, they usually come with scratches ("cleaning marks") or haze or both.
Gone are the days when we traded these lenses for US 300 .... (only 2 years ago ?)
Roland.