Identifying a bulb from a 1969 built light fitting
I've owned a light socket exactly like that one - identical in size to an E27 but totally lacking in internal threads. Mine was simply a manufacturing screwup - the rest of the sockets on the shelf, with the same product number, had the expected internal threads. I got mine all the way home and installed before I noticed the flaw. The store was quite happy to exchange it for a "real" socket that worked.

Replacing that socket with a normal threaded E17 socket is an easy job.