You said, "Finally there was an old Spanish 20 bore sxs with semi-pistol grip, cheek piece, fences, Greener cross bolt. Disc set strikers and provision for sling swivels on the under-rib (Ł250 on the price ticket).
I thought I would check with the book before trying to buy the gun."
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If you are considering this for 9.3x74R conversion, I would feel more comfortable about it if it were proofed for a magnum 20 gauge, which it may be, you didn't say, based on what I previously had told you about my Francotte conversion and similar conversions that I have made. However, such a conversion may very well work on a non magnum 20 gauge, but that is unproven ground, in my experience, as I have never done that, and I know nobody who has.
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You said, "Chapuis 9,3 x74R sxs built on a 28 gauge frame that was as light and responsive as a conductors baton."
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Bear in mind, that this Chapuis 9.3, on a 28 gauge frame, is on a modern day frame made from high strength modern alloy steel, generally many times stronger than common mild steel frames used in the past for classic double guns/rifles, so one cannot directly compare modern high strength alloy steel frames to old classic frames of the past. Many guys overlook this very important issue, since they know no better. Such ignorance can lead to broken frames, during proofing, or worse.