I don't know when they started making mountain bikes, but I had a U-brake model a few years ago, which was probably an '87 or so. This one is 1993 or '94, and it is very happily equipped with an almost complete Shimano Deore LX group.
Finally, and maybe most critically, the front shifter works! I had a moment of terror when after I got home with it, I noticed no clicks in the upshift for the front derailleur. These Shimano rapidfire front shifters were not very durable at all. However, once I got it on the stand, it worked. Once I get a new can of Clean Streak, aka Miracle Sauce, I think it'll click again and work well.
It's made from Tange Infinity tubing, which I don't know much about, and has a set of those great Brahma bars with the bar-ends built in. Also a Kalloy steel seatpost, a Tange headset, and Cheng Shin Ritchey knock-off tires. I'm not totally certain about the tires, but otherwise, this bike is 100% original. It's quite a time capsule really. It has Kuwahara a branded seat and stem, which is just a great touch that old bikes
Which makes me a little sad to strip all the LX parts off it for another bike, but what can you do? My other bike is cooler, so it gets the good parts. It's the harsh reality of vintage bikes - they can't all be Rocky Mountain Thunderbolts fillet brazed by Chris Dekerf. Some - in fact, most - are mass prodcued bikes that didn't always get the respect they deserved.
Even though this is a time capsule, I'll strip it and put it back together with whatever I have left around the shop. It won't be original, but it'll work, and it'll work for a long time. And don't feel bad because I have another bike in a similar state that I'll leave as is.