What is the type of this dc motor?
Can you determine whether those are resistors, or field coils?
If they are field coils, a useful search term is &quot;compound motor&quot;.
These incorporate both series winding and parallel field coils : the combination can flatten the torque curve of the motor (which falls off at high speed with just a series field winding).
They were big news in &quot;Cassells New Technical Educator&quot; (1897 edition) because of their perfectly flat curve, but according to &quot;The Admiralty Handbook of Wireless Telegraphy&quot; (1925 edition) they are pretty much obsolete.
It may be that connecting either field coil or both can give a range of speeds.