Depending on what car the engine was originally installed at would mean that the sequential numbers we can read would have typically been proceeded by the car plant code (F for Dearborn or T for Metuchen) Also, typically there was also the number 0 to identify the year also at the beginning. Example 0T134567
Since we don't (from what I'm reading) what the cars VIN was the letter is in question.
There are a few examples from both plants where the worker did not stamp the full (year, plant and sequential) at the back of the block but these IMHO are not common.
Couple of Dearborn examples from what appears to be around the same time as your car. As you can see the 0F can be stamped lighter than the rest of the numbers in some examples. Maybe take some fine sand paper and clean away the old paint and crud then take another look as a "next step"
Below I added an example of a 70 Metuchen stamped Boss 302 VIN