I believe that the "AC" part of the BNC-55AC is a reference to the ARCAS rocket that inspired the part. Much like most of the two-lettered nosecones seem to match up with the kits that inspired the use of them them (ex. PNC-55EX (Exocet), PNC-55HJ (Honest John), etc.).

I suspect that the single lettered nosecones (PNC-50Y, PNC-55D) may just refer to a series of different shapes that were drawn and lettered, where the designers then chose the ones they liked.

Perhaps if a basic shape was liked, the person who drew it may have been asked to play with that shape and the new derived shapes kept the original letter and got a second letter. From there the best of those were picked (ex. PNC-55AO, PNC-60AH (though the AO, and AH are, I've been told, the same shape, just at different scales).

I guess someone could ask Vern, or any of the other surviving designers.

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Dreaming of making the rockets I dreamed of as a kid (and then some).

"The Guide says there is an art to flying", said Ford, "or rather a knack."
"The knack lies in learning how to throw yourself at the ground and miss."

Launching is Optional... Landing? That Depends on Trees.