[QUOTE=L82M21]Found my build sheet for my 78 on the top of the gas tank on the drivers side. With a flashlight, you should be able to see the edge over the front of the tank. Funny thing is that the window stickers shows a MM4 transmission but the build sheet shows a M20 which was standard in 77. The car build date is 12/7/77. [/QUOTE]
Eric
This is among a few "pecularities" during the ordering/production of the 1978 models and once you understand how the system works, it makes better sense.
RPO MM4 is a generic ordering code for a four-speed transmission but what was called out on the build sheet was dependent on the motor option ordered which would call out the specific tranny type. So, when MM4 and L82 was specified, than the build sheet calls out the M20 Borg Warner four-speed broadcast code of ZU. When MM4 and L48 was specified, the build sheet calls out the economy cast iron Muncie four-speed with a different broadcast code. The M21 when ordered, the build sheet shows a ZW.
In my research, I've studied one L48 M20 and believe the broadcast code to be S6 but unable to confirm via a build sheet.
So the MM4 because it had two tranny options, was linked to motor type which than called out the exact part for that particular installation.