AFAIK, the only replica Sharps models that are NOT proofed for nitro loads are the loose powder or paper cartridge versions. The only warnings about using modern loads in replicas was issued by Harrington & Richardson with regard to their Trapdoor Springfield replica. It was recommended that modern loads, intended for the likes of the Ruger No1 and Marlin underlever, that is to say, jacketed bullets around 405/500gr, should NOT be used in this archaic design, but to stick to 300gr lead loads of around 1200-1300 fps, rather than the ca.2000fps loads used in many modern arms.
In any case, as laid down by the CIP rules, the type of proof is clearly marked on EVERY Italian-made replica, regardless who makes it.
PN = black powder.
PSF = nitro
Also, in the case of black powder-only firearms, the words - FOR BLACK POWDER ONLY - clearly stamped along the barrel.
tac