Hard time following this. So the shaft slid down 8"?? There isn't 8" of cradle. The shaft slid out of the saddle and went 8" down through the DEC housing of the mount and stopped before it dropped out of the mount?
The top of the saddle is solid except for a small bolt hole, so the shaft didn't slide through the saddle?
I'll have to get it drilled, and the little set screw at the opposite end of the housing, which is missing, and the thread size/fine/coarse-configuration is unidentifiable. I tried, that's probably why it's missing; I'll put a "shaft lock" bolt there to stop rotation.
What?? What set screw at the opposite end of the housing? Do you mean the set screw in the opposite end of the DEC shaft which would be for a toe guard bolt and washer? HUH? Put a bolt WHERE to stop rotation of what? The saddle on the shaft or the shaft in the housing or??? The saddle is shown in the first picture, then there is the shaft that goes in it, and then the DEC housing of the mount that the shaft goes through. I'd need pictures to be sure of what is being described.
The shaft is usually a snug fit into the saddle, if not a press fit or even a tighter heat shrink fit. The shaft will still work fine if it slips in and out of the saddle, as long as it is not distinctly sloppy and even sloppy CAN be anchored. One simple thing to do is to make sure the shaft is tightly bottomed all the way into the saddle, then mark the location of that set screw hole. One way to mark the hole is to run the set screw on and off a few times, or get an accurate pencil point in there. Drill a hole in the shaft at the mark, so a longer set screw will anchor INTO the shaft. I would not bother with glue if a shaft is any form of press fit into a saddle, but if the shaft slips in the saddle then I'd add glue in addition to the set screw going into a drilled divot in the shaft.
A bolt going THROUGH the top of the saddle and INTO a threaded hole on the END of the shaft will certainly help hold the shaft into the saddle, but it provides minimal protection against the saddle rotating on the shaft. You need that side screw shown in the first picture to anchor the saddle against rotation ON the shaft.
Ideally, you would have a hole in each end of the shaft. One would be to bolt the shaft into the saddle and the other would be for a toe guard bolt and washer to protect against a weight accidently slipping off. My Caves haven't had toe guard systems but they are a good idea and worthy permanent mod to drill and tap into the shaft end.
Please show a picture of any bolt missing in the DEC mount housing, that would be a shaft lock for DEC rotation. Perhaps a link to the previous thread on this scope?