James,
EO is a prefix code for a 350cc Dirt track engine, c1929. That would make it quite unusual as by far most Dirt Track models were 500cc. Is it OHV?
If the prefix code is instead EQ, then it is a more commonly seen 500/600cc side-valve engine for the S5/S6/T6 models, late 1929 to 1933.
-Doug