Not very well at the moment so I'm making maps.
Anyways, the USPD was the left-wing minority faction that emerged out of the split in the SPD in 1915/1916. The party was only significant in two elections; 1919 and 1920. Can't show 1919 right now for one or two reasons. The party split at the end of 1920 with the majority faction (in terms of members but not in terms of deputies) becoming part of the KPD and the minority faction ('' '') eventually, returning to the SPD. A tiny minority didn't join either party but were never electorally significant.
IIRC, the "U" stands for Unabhängige, which is a weird word.
These results clearly aren't by
Land. What are they by?