NSU model 501T from 1929. This is a heavy 500cc side valve.
Due to the great success of D-rad NSU couldn't stay behind to release a large heavy touring engine. This became the 501T in 1927. This model was made until 1930, then this model got a saddle tank and became the model 501TS. In 1932 NSU merged with D-Rad.
This is a very nice restoration which starts and runs perfectly.