You might find this article of interest at the Corvette Action Center

 

 

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.

 

Ditto.  I was able to get a copy of the dealer invoice for my '80 through Allied-Vaughn.  No complaints.

 

Technically, the invoice was prepared from the information on the build sheet, so the two would be the same with the exception of format and  costs.  The invoice lists prices, shipping and destination charges, etc. and also lists the date the car was scheduled to be shipped from St. Louis and expected delivery date at the dealership.

 

My sweetie was built on 3/3/80, so she has a birthday coming up.