It is a .38 S&W caliber - the only .38 they made in a top break. If the caliber is stamped on the left side of the barrel, it is a modern - smokeless powder gun made after 1910 or so. The SN will be either on the bottom of the trigger guard or under the left grip on the grip strap.
I have one of the hammerless versions in my pocket right now, made in 1915, and it shoots very well for what it was designed to do.
Magtech makes the least expensive and still good quality .38 S&W ammo, and even the old black powder guns can take a ver occasional use with it - the will stretch and get loose with use, though.
To me, they are very interesting old guns, widely used self defense guns for working people of a past era...
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