Larry and John - It sounds like you may be describing different points on the axle collar mounts (aka - spring seat and cap assembly). The stationary hole John describes, I think, is the round opening on the bottom of the lower seat portion that rests on top of the spring and spring locator pin to keep that bracket in a stable position.

I think the "slot" Larry is referring to is the space on the upper side of the lower seat (inside the curved opening) where the axle housing locating tab (I think it's more than a pin) rests. Two different parts with two different functions. That upper side slot is meant to keep the axle position centered L to R in the brackets by the axle housing tabs, but also allows the axle housing to rotate in the collar (formed by the spring seat and cap) letting the torque tube and axle move slightly during operation of the truck.

The spring seat and cap assembly, from what I can recall from previous research, indicated there were actually two different style caps. The rounded kind that you can see in John's picture that does not support overload springs, and the flat style Larry shows that does support top mounted overload springs.

Clear as mud???

~ Dan
1951 Chevy 3 window 3100
Follow this story in the DITY Gallery
"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)