Each drum has four serial numbers.
The body, top cover, spring housing, and the bullet platter. I discovered a significant quantity of drums I sold had three matching numbers. The body, spring housing, and the platter. These parts wear in and mate together. IMHO, having these three parts match are much more important than a matching top cover.
The way some platters are marked and clocked, you'll need to disassemble the drum to verify a match. Many drums have been repaired or rebuilt at the depot level after the platter stop breaks, or springs fatigue.