To OWD's point, as I have read, seen, and heard, you only need the new steel proof mark to shoot "high-performance steel" loads. (Super fast, heavy payloads.) Most in proof standard Nitro proof guns can shoot "standard steel" loads from British and European cartridge makers with no ill effect. Read the article below on this subject from the proof houses. Exceptions are listed in the proof houses' article.
https://www.gwct.org.uk/media/1106151/Proofing.pdfSo that WR gun can shoot standard steel without the new steel proof mark. But not high performance steel (super fast, heavy payloads).
Most pheasant and duck hunters in Britain can shoot British and European standard steel loads in that gun and kill birds. Watch some of the recent driven game videos by Johnny Carter on YouTube (TGS Outdoors) on this subject matter and see for yourself.
There is also a series of videos on TGS Outdoors where Johnny has been following a local duck hunter who shoots high performance steel loads through his standard nitro proof 12 bore Uggie. (Against the advice from the Proof Houses.) It has developed a slight bulge in one barrel, but no other failures so far after four years of regular use on shoreline. The guy still uses it as his primary duck/goose gun with high performance steel loads.
Personally I am not a fan of steel shot. When I shoot non-toxic I shoot factory bismuth or Tungsten Matrix shells in my Benellis and Berettas (mostly RIOs, Kents, and old Bismuth Company shells). And I still hand load bismuth for my older SxSs.