Glenn; I was looking at the Plymouth Owners Club car judging rules last night and remembered reading this:
"The 30U carried a U series engine but to differentiate between the old U engine & the new, engine numbers ran in a different sequence. The 1929U numbers ended at U110,000, while the 30Us U series engine began at engine number U200,001 and ran to U277,000." (pg 22)
I am not an expert like others in the club whom you might consult, but I thought this would be a start for your research.
Good luck, Kim Mc ('29 U biz coupe)