It was a one off bike made to compete against a BMW challenge in the unlimited class of the '73 ISDN. Husky and Maico both created 240 lb over 750cc single cylinder MXers.
http://www.off-road.com/dirtbike/wil..._rs_oct00.html
Did a little research because this did sound vaguely familiar and came up with this link. Weird thing is that Rick "Super Hunky" Siemen was one of the editors at MX mag back then if memory serves and is a principal at this site.
The 501 was an incredibly powerfull bike and most riders could never tap the potential of it back in the early 70's. I had a chance to ride one while stationed in Montana and the power made my yZ360B look like a 125cc. It was a torque monster.
I wonder if the knowledge gained from the one off helped to develop the 501?