So it appears that even though the seat belt holders were not used for 1970, the basic part number 044D90 had provisions for it. The reason may be as simple as this - it was a carry over design from 1969, and the insert that covers the hole is really all that was needed.
The only problem I have with that is the fact that for 1970, both Mustang and Cougar, there were all new part numbers with the D0 prefix, so they were not carry over parts in the true sense of the word. There must have been some other difference(s) that resulted in the new part numbers. The C9 that was cast into the D0 part means that the mold didn't change, or didn't change much. It would be very interesting to compare a known 69 part and a known 70 part side by side to identify the difference(s).