If I was interested in buying it, I'd want a factory letter stating the chambering is factory - no letter = shooter $$$.
It's fairly EZ to rechamber an -06 single-shot to .300 H&H, and the extractor's a no-brainer.
When Browning introduced them in '73, the only chamberings available at first were 22-250, 6mm Rem, 25.06, & .30-06.
The iron-sighted .45-70 didn't come out until later on.
I bought a new tapered octagon barreled .30-06 and a round barrel 6mm, at the time - so, I looked over my options pretty good, at the time (which was early on).
Later, the 7mm Rem Mag became available. It's extractor, available as a part, would be the "no-brainer" I was referring to for a rechambering job from a standard case to a belted magnum.
BTW - My copy of the Browning B-78 Field Service Manual lists only the six chamberings, which I've cited.
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