I'm sure I can remember reading an article about the '93s, which said that they were made for a time in their early production with two different grades of barrel steel, one safe for higher smokeless pressures, and the other made to handle the lower blackpowder pressures, and so marked.
Bear in mind that the 30-30 had just been introduced by Winchester as the first smokeless, high pressure (for those days) cartridge load and the 38-55 was also headed towards a smokeless loading, but most shooting/loading was still done with black-powder.
Marlin marketed their 38-55 rifles in two versions/prices to appeal to both the smokeless shooters, and the black-powder guys who also saved a couple of bucks by buying the slightly cheaper to produce rifle.
If your rifle was mine, I would shoot it with black-powder loads and lead bullets, bearing in mind it's age, and not risk it with modern factory loads which it wasn't intended for.
Due to it's far superior design (who's biased? - not me!) it's considerably easier to field strip for cleaning after using black than a '94 Winchester.
Regards, DaveF.