Mike,

Merkel made all of the big bore Nitro Express calibres on their 323 E and 324 sidelock U/Os but that was a long time ago. Not sure if Westley RIchards made 470s but they did make U/Os with their 425 W-R round even with single triggers for clients who preferred that configuration.

IIRC One of the Christopher Austyn books has a chapter dedicated to big bore U/Os.

Good hunting!