trivia question

[Alan Ruttenberg <alanr@media.mit.edu>]

A friend, (who wished to remain anonymous :) asks:

Perhaps you can forward for me a trivia question I've been trying to
get answered.

I'm pretty sure the phreak magazine "2600" is named after 2.6KHz,
which I think was a tone used for some phone billing system that
got cracked (perhaps Cap'n Crunch's whistle frequency?)

The trivia question is this: can anyone point me to a definitive
reference for what 2600 really was? An ACM article or the particular
phone system or the cracker who exploited it would suffice.

thanks!