“Proofing” at home can be done if someone knows what they’re doing. You need the proper tools to take all the necessary measurements after the “Proof Load.” Personally, I don’t see much value to such a practice, but if it makes you feel all warm and fuzzy inside, then be my guest. But simply having your gun digest a stiff load without checking critical dimensions afterward is a waste of time, and needlessly abusive to the gun.
Bore obstructions, critically damaged guns, bad reloads and wrong ammunition is what causes guns to blow up 99.99% of the time. Most everything else are anomalies. Low pressure loads in Damascus guns is less about safety as it is just not abusing a classic gun. One has to keep in mind that the Damascus barrels aren’t the only old and inferior (by today’s standards) steel used in those guns.
A good visual inspection (provided you know what to look for) followed by mic-ing the bores is probably your best yardstick to determining if an old gun is safe to shoot.
My guess is it fell to .458Win 475gr lathe-turned solid brass PMP projectile. 
Next time they should throw some muuuud on those faces to make those pics more convincing! 
I heard that BIG W is a sensitive fellow. 