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There was one in 243 at h anf f hamilton 6 months ago, with a nice mount and scope. And reasonably priced at around $1200.

The one I got is a 22-250. But still in Europe for now.

You could always have an octogonal thread cap made and timed to aligned with the barrel flats.
the older model B78 and earlier had 24 or so different parts, the more modern productions have that...