Depends on where you're looking. If on the transmission M would indicate the Muncie plant. If on the vehicle ID plate, M would indicate a 3900.
Here's the info from the 55.1 Vehicle Info Kit on serial numbers.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
... and who's to say that the Delco-Remy folks didn't skip the "I" while the people at the engine plant used "I"?
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
The letter I wasn't used in the distributor date codes, so L is November, M is December. As for the motor coded L274 would be Monday 12/27/54.
Distributor date codes J- January B - February C - March D - April E - May F - June G - July H - August J - September K - October L - November M - December
