A few clarifications:
The RPO code is the three digit code. ie - "M20" is the code for the transmission. These codes are still in use today.
The "leading" characters or digits are just dummy placeholders and can be disregarded. ie "04M20" doesn't mean anything and doesn't change anything.
So basically there is no such thing as a "7 digit" RPO code.
The following characters are the "Exception Control Letter". They modify the RPO and provide additional detail, but not down to part number level. Think of them as a "bridge" between the RPO and the specific part number required.
https://67-72chevytrucks.com/vboard/...hlight=corvair
https://chapters.corvair.org/corvana...ndTrimECLs.php
So, for example, you might have an engine code with a suffix like LS9AA, which distinguishes between a particular engine with a standard clutch vs an optional HD clutch.
Or, for a transmission, M20AB might be a different speedo gear than M20AC, for use with a different rear axle.
Apparently this extra definition wasn't worth the effort, since by the time I started in 1979 the ECL had fallen into disuse. SPIDs from that timeframe carry another set of dummy placeholders in those spots or are not printed at all.
Unfortunately, these codes were not saved for posterity and so it is not likely that they can be replicated accurately. If you can find another SPID with the same content you can use those, or you can make something up because there won't be anyone or any way to prove differently.
K