That's a nice rifle. Let me present my thoughts on the question at hand.

It appears the rifle was re-proofed at the London proof house in 1982 (if I'm reading the stamp correctly). I'm assuming the re-work to 9.3x72 was done just prior, and the re-proof was occasioned by the re-work to the new chambering.

In 1982 the "preferred" choice would have been 9.3x74R just as it is now. That being the case, it is hard to imagine that the gunmaker who converted this rifle overlooked the larger round and foolishly chose the obsolete round. I believe the choice was made for a reason, and I believe the reason was the professional judgment that the x74 round would be too much for this rifle. There would be no other logic in chosing the obsolete round.

The same logic applies today. I would leave well enough alone. I certainly would not convert to x74 with the idea of using non-standard light loads. The next owner might not know the protocol.

I see the rifle lacks a third fastener. That may have been a factor in the choice for the lower intensity round.

In the end, if the discussions on this forum do not satisfy you, the best course of action is to consult Holland & Holland. The maker can give you expert advice to be relied upon. Nobody here has that expertise.

As stated, I would keep it and enjoy it "as is". There's plenty there to enjoy.

Curl

P.S. My warm welcome to you as a new poster on the forums! Please join in often.