Not quite right.

The "O"-75 part is on another line than the 48378, right?

If so, then it's the standard two-part model number, where the front part is the body size and the back part is the style.

Now the "O", pronounced "aught," (as in "naught") is a valid body size, small by today's standards, and noticeably smaller than a standard dreadnaught.

The 75 body style, however, is a problem. It doesn't match up with the "0" size.

(My reference source right now is "The Martin Book: A complete history of Martin Guitars" by Walter Carter. My edition goes through 1994, and a new updated one was recently released.)

If your serial number is 5 digits long beginning with 48, then your Martin was made in 1931. There weren't any 75 styles made with "O" -sized bodies, let alone in 1931.

There was a GT-75, but it was an electric archtop double-cutaway made from 1965 to 1967.

There was a D-76, but that was ony made for 1976, to honor the US Bicentennial.

Can you describe your guitar and maybe measure it? One dimension that I could work with would be across the width of the lower bout.   (If you've never heard of that before, that's the wide part of the guitar opposite the soundhole from the neck and fretboard.)

Also, if your guitar really is 3/4's of a century old, it can be extremely to read those hammered-in digits the right way. I bought a 1928 model 2-17 this summer, and it took me at least 3 tries and one of those flashlights that send light around a bend in order to be sure I was reading it correctly.