DRJ,
I doubt that the proof pressure would be above 30% for rifles chambered for higher pressure cartridges.

I think that the "30% to 50%" does not mean that ALL rifled arms are subjected to proofing above 30%.

I think it means that lower pressure cartridges like for example handgun centerfire, might be proofed at the higher end of the 30-50%, and that higher pressure cartridges like .375 H&H etc might be proofed at the lower end.
Once you are dealing with cartridges producing pressures above 60,000psi, a proof load of plus 30% is going to be pushing the brass cartridge case close to or beyond its limits.
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4seventy,
I certainly agree with you on above points. However, as I mentioned in my piece above, my three Sabatti rifles all came with "official" factory proof certificates that clearly indicates that proof was done between 30% and 50% over service pressure, which means exactly as stated, not at 25% over; not even at 30% over, but between 30% and 50% over, 25% over isn't mentioned at all on certificate, so the age of book "The Standard Directory of Proof Marks" has no valid bearing anyway, considering details mentioned on proof certificates that came with new rifles. Reading some "facts" from some publication you don't identify, doesn't always mean it is true. Further, a few days ago, just to verify, to confirm the facts, I had a friend in Italy contact firm in Italy by phone, asking them what proof pressures are presently used, and they confirmed only that "between 30% and 50% over service pressure is used today."