That is partially what my theory is.
The package coded changes the price class but the higher class may not necessarily be recorded in the VIN. See the DM (M= medium) VIN but the DE (E= economy) reference with the A71 code. The A51 and A52 cars get PH VINs but PP and PS (Premium and Special) designations; or 'lower' VIN price classes.
Here's why this all matters.....
It's helps to be somewhat familiar with the underlying Sales Reports, the source of which production and option distribution numbers are derived.
GG's white book, a common source for production figures, does not list any numbers for the 70 DM Polara Custom cars.
IF production numbers are accounted for by the option code number, say A74 and not by the DM VIN, then the production tally may appear on as a line item on the report and not as a column heading tally.
C body sales reports don't seem to be as prevalent as A, B or E body cars. IF the full 70 Dodge sales report even exists, was the page contains the A section codes missing from the copy GG had meaning he couldn't report the numbers?
Knowing where and how to look at the reports would clarify production figures and answer other questions regarding the A** option codes.
This is one of the reasons why the reported totals (see the other thread on the DE cars) might not balance. IF you are trying to account for cars via a VIN code and not a package code, you are looking at and interpreting the data incorrectly.
Of course, I'd love to see the full 70 Dodge report to clarify all of this.
I thought maybe the A74 code might be new for the 70 models so I looked it up on the Hamtramck site. There is no 74 code listed there. Was it still used in 71? There was no ording information listed at all for 71. I looked at 72 and found nothing there either. Very strange. Like mentioned before, could it have been a mis-stamp? Of the A74 cars that are still around, what options do they have in common that may fit into a option group with a number similar to A74. Just thinking out loud.
Note: the Hamtramck site information is only as complete as the original paperwork that has been turned in and posted. The 1970 Hamtramck Registry - The 1970 Dodge Division Code Guide - Dodge Full Size