Mike,
I have used piano wire rings to get the headspace correct. using thin rim brass.
I believe Bertram has a run of these cases. You could check with their USA distributor. Cost is about 2x what it had been but you get thick rim brass. My recommendation is to anneal the cases you get. It isn't fun to have splits in very expensive cases fired under moderate loads.
I foolishly sold a nice set of dies and then bought a nice WR DR in 50/450 #1. It took several years, looking on eBay to find another set of dies. They show up now and then.
