Tony,
The "scratched letters" were done by foundry workers and DO NOT appear on factory blueprints for the blocks. Raw castings are very generic looking and the letter scratched in the bellhousing area was done to make ID'ing the different blocks easier. The foundry was HUGE and there were many "stations" the blocks went to so a simple in house ID method was developed. This happened on "some" small block engines as well. Without first hand information from someone who worked there at the time ( tough to find one 50 years after the fact) anything is a guess at best. I am going to the Cobra Jet reunion this weekend and there will be retired Ford engineers there . I will ask to see if anyone of them has any insight. Besides the "special" guess , I offer "stationary" as in industrial pump motors that were "extremely" popular at the time. BUT that is a guess too.
Randy