The 1996 M2 hardtail takes a 30.9mm seat post. Bow Cycle was able to double confirm this, and on top of that, they had two lengths of Thomson posts available in stock - but these were the only two.
I asked them if they'd hold them until I dropped by after work, I made it there just before closing. I eyeballed the length, and elected to go with the longer one. If you're wondering how a blog post can drag on about a simple seat post, let me assure you they're well worth it.
Thomson seat posts are as near to perfect as I know. The one I acquired tonight is beautifully black. It doesn't require the squeaky, creaky shim (aka inelegant bandaid solution) that the prior post did. Squeaky, creaky shims squeak and creak when pedaling, which is easily mistaken for other moving parts making sounds, and is unacceptably annoying. The pride surrounding the rebirth of my Stumpjumper does not need to be dampened by creaking.
The clamping device clamps the saddle rails. I contrast this to the prior post's clamping device that merely tried to clamp the rails, yet couldn't try hard enough to properly accomplish the task. The saddle rails don't slide though the clamp. The angle is infinitely adjustable.
Riding home with the silent sturdiness of the Thomson post supporting me was the highlight of my day.