[Dan Case]

So far, our group has not figured out the letters other than the sequential serial number.

So far, many known original Cobra blocks have a "P" stamped on one of the two tooling bosses on the left hand side. Many engines are stamped either "PA" or "PB". Sometimes the P is on the number pad and sometimes not. When found alone the P can be rotated just about any direction. Sometimes SAI included the alpha characters in the engine number on the sales papers and sometimes they didn't.

Original foot box data plates got stamped with, in addition to the numeric serial number, no alpha letters, PA, PB, P, AB, BA, and A that I have photo documented so far.

The alpha letters must have meant something to somebody on the engine machining and assembly lines but beyond that it is just speculation.