391 was a FT engine. But based on the 390 4.05 bore block. It used a 3.794 stroke forged steel crank with a larger snout dia than the std FE cranks. Heads had big chambers with small valves with the heat crossover in the center not offset like the FE's. I've only seen a few different foundry identifiers. 352 and the reverse 105. All of the old 406's I had did not have the 352 on the upper left part of the block. They had "HP" on the side and rear of the block. And by the cam and crank saddles on the inside. I have seen a few builders that stamped the CID on the outside of the block. But nothing like that from the factory that I've seen cast in. And I sure have not seen it all some 30 years later. New things pop up all the time on these old engines.