You misunderstand me, Jim.
The OP's gun.
Was it originally chambered for .38 Long Colt and then gunsmith converted by reaming the cylinder?
The barrel markings could possibly indicate that.
If it WAS originally chambered in .38 LC, but then post-factory altered, I do not believe it would be wise to fire it with .38 Special ammunition of any flavor.
I'm not particularly worried about a sudden catastrophic failure. I'm more worried about the possibility of greatly accelerated wear trashing what could be a fine handgun.
"FWIW, the S&W Model 1899 M&P cylinder is actually smaller in diameter than the Colt Army & Navy cylinder, 1.44" to 1.52"."
And, for what it's worth, without knowing anything about the properties of the steel that was used, trying to gauge inherent strength on the diameter of the cylinder alone isn't particularly wise.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.