Crazy barn find generator
I suspect it is a dynamotor to provide high voltages for a mobile, vacuum tube-based radio station of one or another kind, from just a car battery voltage. It is similar by appearance to the dynamotor seen in that Wikipedia article. It probably has military origins and powered up something serious and with transmitter. Just for a car radio, seems overkill. This also would explain why it is little used.
It is an interesting piece of technology but I really cannot think anything useful  you could do in these days with such a converter. If to power a nostalgic tube amp, it is probably for that too noisy and the amp should come with its own transformer. Careful with high voltages it produces.