Cast ironware, through the years, has been produced by numerous makers. Back in the day, in fact, every blacksmith produced cookware as part of his general workload.
Commercial (that is to say, factory-made) was produced by numerous foundries, most of whom did not put on a makers mark. The "5" on yours could have been a maker's code, but doesn't refer to much else. One of my 8" skillets, for instance, says "made in U.S.A" and has an "02" code. No other markings.
So, first off, I wouldn't worry about who made it. It obviously did the job for your Mom, so is worth keeping and using.
Question: Does it really have to be reseasoned? Providing there is nothing burned-on or crusted on, why would you want to remove the cure your Mom spent years building?
All I would do, initially, is run straight hot water and use a scrub brush. That will remove any dust etc. that's built up through the years, and any loose or scaly debris. Then examine the piece. If the black finish is smooth, and there's no obvious rust, then dry the skillet, wipe on a thin coating of shortning, and you're ready to use it.
After each use, clean it the same way. That is, hot water only (never, ever, use soap on cast iron!!!), a brush if needed, dry it, and reapply a thin layer of shortening.
If the piece actually needs recuring, that's something else. Let us know, and I'll direct you to some sites that explain how to do that in detail.