FCinCA:
This is just my opinion but, since the Diamondback in question was originally designed to chamber the .38 special cartridge, there possibly wouldn't be any problem with shooting these! But, how could you be certain nothing would go wrong? You can't be! However, since this gun is mismarked, there's always a chance that sombody is going to take the (.357 &) caliber marking stamped on the barrel literally and fire the .357 round and very probably will suffer serious consequences!
Another thing to consider, is that since this Diamondback had been converted, nobody would want to buy this revolver-unless they were duped into believing that this gun had actually been chambered by Colt's for the .357 cartridge! Technically, this gun has been ruined from doing this conversion! I wanted to kid myself into thinking that this Diamondback would have been a nice novelty to have? But, just because someone had stamped an incorrect .357 marking on this gun, doesn't make this a good novelty gun to have! Only a damn fool would have modified a revolver like this that Colt's themselves had never chosen to chamber their Diamondback in the .357 magnum round! Why? Because this frame and the metallurgy, just would not stand the power of this .357 magnum round! Don't you think that Colt's would have chambered these Diamondback in the .357 magnum caliber long ago-if this were possible(And, if it would have been safe to do this?)? Years later, Colt finally had introduced the updated stainless Magnum Carry revolver that was based upon the "D" frame-but, these were beefed up wherever it was necessary, to make them plenty strong enough to chamber the .357 magnum cartridge!
I know exactly what you are thinking-because I was thinking exactly the same thing! I first thought, well, I'll just keep this Diamondback in question, and fire only .38 special's? But, I knew that I'd be buying an unsafe gun that I could never sell to anyone-because if they had ever fired this gun and were seriously injured(Or worse, killed?)then I'd definately be held liable for selling an unsafe gun! There was also the matter of the steep $1,200 price tag(For a gun that was worth around $700, if it was original and unmodified!)and I had presented this question to the gun dealer that was selling this gun! He wanted to believe the hand written note inside the gun box that had stated that this was a rare Diamondback(Simply not true!)! But I do know that this gun had been up for sale for up to a year before it somehow got sold or traded to another person or gun dealer! Everyone fell for the mintness of this Diamondback-and they wanted it! However, nobody for a long time wanted to take a chance in believing the phony(Added on)barrel markings for the .357 round! I sweated out the idea of buying this Diamondback for a long time! I wanted it! But, I finally came to my senses and passed on this beautiful Diamondback that some jerk had ruined! And to wonder why? These .38 special Diamondback's are neat revolvers-and, are hard to find here in CA(Especially in pristine new condition)! I would have given my right arm to own that Diamondback! But, it was only right for me to back away from buying this revolver! This was a very painful experience for me! I think that the only reason that this Diamondback had been converted to .357, was just for the purpose of conning somebody into paying big $$$ for this phony rare gun! Otherwise, it would have been shot!