Found this description:

Bullet Dia. .423"
Neck Dia. .452"
Base Dia. .485"
Rim Dia. .541"
O.A.L. 2.780"
Case Lgth. 2.047"
Rim Thck. .043"

Further - the 10.75 X 65R Collath has the same (close at .487" base) and J.J. Donnely's book says to use .45 Basic brass & turn the rim to .541" & shorten the case, etc, etc.
Not sure how that would work, as the basic brass I have, measures .497" at the base.