It was on the seat tube close to the bottom bracket. It is 6A267. Seems a bit short, but that is what is there. Thanks so much for all the info everyone, it is so helpful. I will post before/after pics when I fix it up and try to figure out how to repaint it.

Should I leave the old decals? Or get rid of them and just repaint the entire frame? They are kinda ratty but otherwise how would you know it is a Centurion?