Macintosh case signatures

Hmm, I have a pair of Mac SE's here, and was a bit surprised to find that one of them has signatures in the rear of the case, but the other one doesn't; does anyone know what dictated whether a machine had the signatures or not? (I was actually surprised to see signatures at all, I thought that went out with earlier models) Anyway, the one with signatures is a SE with a single 800K drive and 20HSC hard disk, last IC date I can see week 25 of 1988. There's also a sticker on the chassis saying July 30th 1988. The one without is a SE with dual 800K drives and a Quantum drive on a bracket (which was possibly added later), last IC date I can see being week 01 of 1988. In other words, what seems to be the later machine has them, and the earlier one doesn't. Possibly cases from different plants, where one had changed their tooling and one hadn't? Or maybe the "later" machine is actually an earlier build but has had a system board replacement (with the date sticker being the date of repair)? Curious, anyway... cheers Jules