Steve,
Double proof would have had three crossed wheat with crown marks, and would have been 20,500psi. Triple proof would have had four marks, and would have been 27,000psi, proofed with black powder. There exist Ideals proofed at the higher level. I'd still run low pressure ammunition in one proofed like that.
All I'm saying is it is an old gun, and what level it was proofed to 109 years ago might be a secondary consideration to using ammunition that would help keep it around for 100 more.
Best,
Ted
