"Belgian revolver Warnant M-1870." I was searching Warrant. That makes more sense. Searching 'Warnant' turns up a great deal of info. Apparently the Warnant lads were a busy bunch. Still not a shooter though.
"...a British crown proof mark..." Belgian. I'm thinking the T.A. & C might be the American importer and the 500 the calibre vs the S/N.
It'd be one imported Stateside, probably a low cost thing given the pay scale of teachers then. Awfully decent of whoever owned it before to send that scrap of paper. Doesn't increase the value any, but does give provenance.
"...Boothroyd's list of British Gun makers..." Needs to be a list of Belgian makers. There were literally hundreds of small smithy shop operating in and around Liège, Belgium, prior to W.W. I. Most of which went out of business due to bombs and shells falling all over the place.
.500 Tranter dimensions.
Bullet diam. 0.520 - case length 0.87 - rim diam. 0.594 - neck diam. 0.520 - base diam. 0.528
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