Hey there, just a lurker here, sorry if I mess anything up. Kudos to all here for the wiki. It's an invaluable resource, so I hope I can give something back with this thread.
So, I've got this TI-99/4A:
When I read that it can come with several different switches, I was curious which one mine had. I could see looking through the vents that it wasn't the Stackpole grid type, so I decided to open it up a bit to take a better look. (Sorry, this isn't a complete disassembly, that looked like it would be a serious job.).
Now here's the interesting part. This one, as it turns out, uses Futaba sealed linear switches, which seem to have very little coverage here or anywhere else online.
The wiki only specifies that the 4A can come with Futaba MD, these switches' big brother. Looking at the references, I notice neither actually shows the bottom housing of the switches, so maybe that caused confusion? (Edit: Nope, the one on the Nightfall Crew site definitely has MD.)
Anyway, time for a couple more pics, here's the latching Alpha Lock switch next to a normal one. It's black instead of the usual milky white.
As you can see in the first picture of the switches, the spacebar is stabilised with a wire that hooks into two dummy switch housings at either end. Here's one of those.
And here's the underside of the PCB. Notice that it's a slightly different design from the other Futaba TIs in the wiki sources. It lacks the Texas Instruments, Futaba and Sejin Electron logos.
An upright picture for good measure:
The caps are held on far too tight to get them off by hand, so I can't show the tops of the switches, sorry.
Anyway, if you're wondering what these switches feel like, here you go: This is, by a considerable margin, the worst-feeling keyboard I have ever encountered. Every key has a horrible, abrasive resistance that is so strong, it slows down the speed you can press them down. This is allegedly a linear switch, but the friction makes it more of a struggling stop-start than a linear motion. It blends with the binding, which occurs on any press which is just even slightly away from the centre or without a perfect downward motion. The travel is disconcertingly short, which combined with the ludicrous keyfeel makes it feel quite uncertain if a key even pressed if you try typing anything. After a press, the keys usually feel like they're in no hurry to rise back up, often returning with a slow judder as they're held back by the friction. Some can even just stay down at the bottom for a second. The feel of the latching switch is identical to the others, except of course that it locks if you apply extra pressure after bottoming out. It's a very separate action from the initial press.
I can't tell how much of that is due to condition, of course. When I got this thing, it had just been rained on, but none of the rest of it looks rusted at all (except for one tiny spot on the RF shield.), and the switches feel more or less perfectly consistent(ly awful).
Anyway, that's it, I hope this thread can be of some value for the pages on the TI-99/4A and/or sealed linear switch. Feel free to use the photos there if you want.