I've owned at least a half dozen Model Ms of various vintages and 3 SSKs (87, 91, and 94). I'm now down to two of each. They rest have gone to Geekhackers. I also have an M13 and 2 Unicomps. The feel of them varies quite a lot. As rootwyrm suggests, wear has a lot more to do with the feel than anything else. The back plate does affect the sound, but not the feel so much. The springs do wear over time. Sometimes they become slightly deformed or wavy. Sometimes they corrode to the point where the hammers get coated in crud and jam up. Even when they don't break, the plastic rivets relax over time and the "snap...clack-boing" slowly becomes a "smack...thwack-boioioing". Performing a bolt mod generally reverts this, though sometimes a few springs need to be changed. I've never had to replace more than a few bad ones.
Yeah, the mat is a significant contributor too. But moreso the plate, believe me. I can tell the difference between a beige and black barrel plate from feel alone, same for the gold, steel and thin plates. I have a bit more trouble telling the mats apart except by sound. I've honestly never had to do any major spring or hammer replacements - anything with broken springs was already too far gone to bother repairing. (Once the barrel plate's busted, it's scrap.)
I've restored a few clunky-sounding M's to their snappy glory this way. I've also replaced the rubber mats on a couple of them (they also deform and cause double-strokes or intermittent keys). Using the Unicomp mat really makes them feel snappy. My '94 SSK is this way.
Yeah, the Unicomp mat appears to be thinner. Which obviously, is not a bad thing, as long as you don't compromise the durability. (You
can do such things these days.) It's when you compromise the durability of the part that it becomes a seriously problem. The mat is basically the only thing between you and effectively direct plate contact though. Unfortunately, that would result in the membrane getting worn through over time. Not really a good thing, that.
The Unicomp boards and the M13 are very similar. They both have a slightly-scratchy movement, a sharp snap, and very little ringing (boing). I've had the M13 apart and it seems to have the same thin backplate as the Unicomps. I don't think the M13 saw a lot of use before I got it. My 60G3571 I bought new in 1994. It's my daily driver at work, so I've felt it get looser and looser over time. It also got a grease mod along the way. The key caps are fairly polished. It probably needs a bolt mod, but it hasn't acted up so I leave it alone. I have replaced a couple of springs on it.
The M13 and Unicomp EnduraPro are completely different mechanically, actually, which is the irony of it. In fact, Unicomp had the tooling for the original M13 - they were made exclusively by MaxiSwitch and Lexmark. But the EnduraPro is based on the 'compact' chassis. (Which is why I'm quick to point out that the chassis doesn't really matter unless it's damaged. There's your proof, right there.) By the M13, all the parts had been standardized but the bolt-on of the trackpoint changes the mechanical characteristics of the barrel plate sufficiently to give it a slightly duller feeling.
The M13 definitely should not have scratchy movement though. In fact, I've never had a scratchy M13 even in bad shape. They have a sharper tone generally, largely due to the chassis, barrel plate and controller changes. It affects sound NOT feel. Plus the added heat of the M13's controller versus the M's controller and M5's controller is known to cause thermal cycling of the backplate. Ironically, the gold plate seems to be designed for greater thermal stability. TBH, sounds like a bad batch of caps though on the M13, or excessive thermal cycling. Remember that a lot of M13's lived in server rooms hooked to KVMs, often on top of hot servers or in exhaust path. So repeated 95F-70F cycling isn't too uncommon.
The only trend I've noted is that the older boards seem to have, on average, slightly heavier springs. I still have an old '87 M that feels really heavy compared to other boards I have...including the brand-new Unicomps. The used M's out there are like a box of chocolates.
They very much are, yes. I know the older springs, they'd favor tolerance over but not under. They usually went 80cN+ there, but they also wear very differently.. I have an '88 3151 here that I particularly dislike and probably won't do a controller swap on. There's no snappiness, just mush and very high resistance. And I've had it apart - it's in as perfect shape as any 3151 you will EVER see in your life, without question. In fact, other than grime on the keycaps, it is pristine. The chassis is ruined sadly, the plate post on the right side was just too damn brittle, but the plate and barrels are just flawless. Not a single broken rivet on it either. Zero corrosion on the springs - they're still shiny as hell. If there's any lubricant in there, it's WD40. So I'm honestly at a loss as to why it feels so mush. I've never seen it before.
