>>11915729
Ah. Must've been a 3rd party battery pack. There's no reason Atari would make an accessory that would be compatible with the competition (unless they're just that desperate for additional sales.)
I had this one for my Lynx. And yes, that's a shoulder-strap on the thing. It even has a belt-clip which should be a comical sight.

A friend did one of his jank mods, soldering alligator clips to the power connection and running the Lynx off a camcorder battery.