How old is the gun?
What make, what model?
Spanish copies of S&W revolvers from the first couple of decades of the 20th century were notorious for being marked like this.
I saw one that said, essentially, the best cartridges are those that fit.
Unfortunately, it was one of those that would take not only .38 Short and Long Colt, but also .38 Special, and most horrifingly of all, .357 Magnum.