5N is only for the Kansas and Pontiac plants, Fremont and Baltimore were Chevy controlled and did not use group numbers on the data plate. Group 5 was accessory codes and has nothing to do with engine/trans codes see the info from John V.
http://forums.maxperformanceinc.com/...+64+trim+plate
John V. has also done the research on the M21 close ratio and there are many errors as he has only documented one M21 car. Most 3.90 gear cars had the code W wide ratio M20 trans in 1964. PHS documents still use the incorrect assumption that 3.90 cars got M21 trans automatically. They miss things and make assumptions, mine they decoded as a 3.23 gear car, but the build sheet documents from PHS code it out as a 3.55 car. Code 9 is for the different tooth code in the transmission for a 3.90 gear setup, the gear tooth count for the speedo in the M20 for use in an 3.90 geared car so the speedo reads properly. Code 8 was the M21 code and only had the one tooth count setup as it was meant only for the 3.90 geared cars.
3 speed and automatic cars got no trans stamp code on the engine. The W, 8, or 9 was stamped on the block so the proper engine/trans got installed with proper speedo calibration with the correct rear end. My engine that was built sometime in November and installed in a late January/early February car was no W code stamped on the block