You've got great local machine shops....save a buck, get them to order up a piloted counterbore cutter (HSS is cool, you don't need carbide), ....fit it to an existing ( you've probably got the thing) set of guts...with a "stretched" base.
That thing at U.S.$ 200, will be pretty $$$$$$ down under, methinks.
All else fails, I'll build and send you a complete unit, or the plans, or the guts, or whatever.......
Happens that I'm helping a guy from Tombstone wake up a Peddersoli in that infamous chambering.
He got Norma brass, from Gullo (BuffArms).....awesome stuff, but costin'
more than good whiskey.
Big old cartridge, that.