It's one of those abbreviations that's been around a while. Young people may not know about that one. We old people sometimes use that and other antiquated abbreviations like 1st,2nd, 3rd, and 4th for first, second, third, and fourth. You might see that in patterns every so often.
Sometimes they would abbreviate names, too. Like Robt for Robert, Jms for James, Wm for William, Chas for Charles. Married women back in the 50s and 60s never went by their first names. It was Mrs. James Stewart, for example. Or when writing a letter, Mrs. Jms Stewart. The Women's Liberation movement changed all that. Trivia and I'm getting off track here.