I have been wondering whether the early version of the M-122 shell would work, I need one, too.
I bought the first (nice) F-122 from Ocean Tech and did not realize how lucky I was to get one that was in good shape throughout. Then I bought a lot of 4 F-122s from Rawko (5 minutes from my office) and they were wrecked. All 4 cases (shells) were broken in some way, usually diagonal breaks from the bottom corners of the key openings out the bottom of the long thin plastic strip at the front. Careful repair jobs using "JB Weld" did not always work and were the wrong color, of course.
There were only 2 spacebars, unfortunately not interchangeable with Ms because the F wire stabilizer is a smaller gauge. 80%-90% of the keys were there, all-in-all it was not a bad buy, but less than I expected.
Big disappointment was the rubber mats. 2 completely disintegrated before I got them off. I did one dry, and soaked one for an hour in hopes that maybe it was glued down with sugar or soda (same thing) and that was maybe a marginal improvement but still did not work. The other 2 came off with a thin rubber layer intact, but the foam was completely deteriorated into a horrible sticky granular mess. One day I will get that hole punch and make a couple of fresh mats.
I have 2 of these F-122s now, one raw and undamped, and one flossed and padded. The raw one has a beautiful clear ringing sound that is great, but I have a wife and 2 kids ......
Padding the case was done in several parts. To partially make up for the missing foam, I spray painted the backplate with several layers of rubber coating, an industrial product made for tools handles and waterproofing. I cut a felt mat to fit the bottom of the metal pan case bottom. I used that rough type of electrician's tape all around the interior assembly and where it contacted the outer case. After I screwed the interior assembly to the case bottom, I tore the polyester pillow fluff into golf ball sized pieces and pushed it into the space between the parts with a chopstick. I used at least a dozen of those which compressed to very little volume. No pictures because it is embarrassingly low-tech and ghetto! I certainly HOPE you don't see it in the finished product!
I thought that I had posted a picture of my bolt locations. I will edit this post later today to show my new scheme. Did it make a difference? I think that it did. The whole bad-ass board is as tight as a drum, and feels like a Sherman tank, but at the cost of some overtones. The feel and click are completely intact.
< Photo deleted - see post below for corrected layout. >
This is my 2nd try. Good thing I had an extra board to experiment with. Ended up moving holes #2 and #3 up to the smaller "throat" because of conflicts with the hammer plates in the number row. Moved #4 and #7 up one row for more clearance with the bottom pan. Moved #9 up one row, because of a conflict with the tab that holds the wire stabilizer. Be careful when you locate #5 because there is an unused hole in the metal plate very near there, and remember to keep away from the wire stabilizer tabs in general.
I used far less bolts than on a Model M, for one reason because there is no room in the middle of the letter keys where you might really want them. I used larger bolts (4-40 American size) and the ones near the bottom (front, spacebar side, whatever you call it) could only be 1/2" long with no washers because there is not much clearance.
From mid-way up (back, whatever) the interior assembly curves away from the case bottom so there was plenty of room for 3/4" bolts and I sandwiched in washers, including rubber washers.
I felt compelled to leave the tabs and slots alone, since that is the alignment process. After slipping Fs apart at least a dozen times, it is getting easier, but it is still a tedious pain. If I intended to do this much more, I might be inclined to get some scrap lumber and build some sort of jig, but then there are the XTs and ATs in addition to the 122s ......
Those tabs that were bent over, to keep everything in place, well, you don't need to do that any more, at least. That was a weird idea anyway, probably makes sense in a factory but surely not intended for humans to have to mess with regularly. At least after you break it free for the first time in nearly 3 decades, it slides on and off a bit easier when it goes back together.
I will be very curious to see how the paint/dye job holds up over time.
Good Luck !