Trivia, what,s this recess for on my rear axle ??

[34 GAZ]

I have a decent collection of rear axles but only one with this 1 inch wide groove on the drivers side . Top side is pretty deep and runs to the normal smooth outer curve at the bottom of the axle. Other side is normal smooth curve all the way around. Looks like a recess for a strap. Axle had 1940 backing plates. 3rd pic is the smooth side.