I've only seen the thicker ones and mine have 4 square headed rods for tightening the adjustments. They seem to have been available from 1970 -73. The catalogs don't seem to show a thinner version so maybe it's a stencil version not used on Pearl branded drums?...and John has the answer for you while i was typing
Oh - are you guys referring specifically to the bottom shaft that goes into the bass drum? Yeah, I have never seen or had that Pearl mounting with the six sided pieces that closes in on the bottom shaft.