I once asked the same question (and others) to the man himself and I got the following response which gives a little trivia on the names of BW bows:
"Back in 1982 Ken Beck bought Black Widow Bows. It was the 25th anniversary of the company and he designed his first Black Widow, the MA. MA stood for Maple Anniversary because it was made of Maple and it was the 25th Anniversary.
Things kind of grew from there. His next takedown model was a shorter version of the MA and he called it the SA (Short Anniversary).
In 2003 we started building our bows with our CNC machines and we added the P in front of MA and SA. P standing for Precision.
In 2004 we began producing the older TF (Target & Field) one piece bows in the milling machine creating the PTF.
In 2007 we had our 50th Anniversary so another anniversary bow was born, the PA (Precision Anniversary). So now we have the PMA and PSA models along with the PSR (Precision Short Recurve) and PL (Precision Longbow), PA and PTF models.
Last year (2010) we came out with another new model, the PCH (Precision Compact Hunter) which has even a shorter riser."