According to Wikipedia,
http://en.wikipedia.org/wiki/Parker_51,
the Parker pen company introduced their model 51 in 1941.

The article says,
quote

The pen's resemblance to the sleek fuselage of the P-51 Mustang, a fighter
plane used extensively during the war, had no bearing on its name; but
Parker took advantage of the coincidence by comparing the pen and the plane
in its advertising. Additionally, a pilot who is suspected of falsifying
flight records in his logbook in order to overstate his actual experience is
said to have logged "P-51 hours," relying on the ambiguity of the term
"P-51" to avoid directly confronting the suspect.

/quote

That's the story I've heard.

"Jay Honeck" wrote in message
oups.com...
btw, for us old farts, that is known as a "P51 Annual"...

'Splain, please?