I'm thinking the dropouts might be specific to an internally geared hub or some early type of derailleur. Most high end modern bicycles have a derailleur hanger on the right rear dropout; and these haven't changed much in the last 40 or 50 years. Before that, there were a lot of innovative designs, most of which were produced only for a few years.

To be sure, I don't know what yours is, but I think it must be from before the 60's.

Very nice, too. The lugs are beautiful.