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Thank you Bill.  Please see my comments below:
David

1) May be a filter on the outside of the top glass. This often has dropped off as the glue gives way. Scratch it and see.

I touched the top with the tip of a craft knife and attempted to scratch it.  It felt and sounded resinous, and it spinned.  The bottom on the other hand sounded like glass.  The top has indeed some kind of semi-opaque diffuser.  I'm going to leave it in place for the moment.  Perhaps it's an improvement.  I won't know until I get my bubble.

2) Use an Allen key and grind a square on the end. There is a steel ball underneath the screw.

Funnily enough, there is such a ground down Allen key in the box.  I started to try it before I sent my post, but I got that kind of feeling not to try too hard in case I really ruined the the grub screw (It might be glued in).  It was going to be my solution of last resort. 

3) The light path does not traverse the bubble unit, so the refractive index of the original xylene (about 1.5) is not relevant. The original seals were lead washers, so you could safely replace the xylene with naphtha (lighter fluid) or alcohol. If the seals have been replaced with ordinary O rings, use alcohol. If shellac has been used to reseal the glasses, don't use alcohol, in which it is soluble.  Leave a bubble about 3 mm in diameter.

By alcohol, I presume you mean the bio ethanol.  I use it in TIKI's stove, because in the UK methyl alcohol has purple dye and pyridine in it to make it undrinkable; It also stinks when you cook with it and bungs up the burners.  Or did you mean whiskey? 

4) The drum is retained by two pegs that have been split longitudinally. Open out the splits a little, but make sure that the disc goes fully home. The key that passes through a keyway in the disc MUST project beyond the surface of the disc or the clutch will not operate correctly and you risk stripping the toothed sector.

Why didn't I think of that? I slipped ny craft knife blade down the splits to widen them; problem solved.

 I write in my restoration manual (which is still available...): The hub carries three pins on its face, two narrow ones that are split longitudinally so they engage firmly with holes in the face of the marker disc, and a shorter, broader one that engages with a hole in the underside of the face of the disc. In each type, the key projects from the bush. In neither type can the marker disc be attached unless the key and the large pin are aligned. When the disc is properly seated, the key projects from the bush beyond the keyway in the marker disc, but unfortunately, it is possible for the disc to appear to be properly seated when the key is still within the keyway. The official manual is silent about this very important point. In this position, the clutch cannot slip and this probably accounts for the many instruments that have teeth missing from the rack. Bill Morris  Pukenui  New Zealand

I was intending to buy your MkIX book when I got a round tuit, but the A12 has recently taken priority.  I will contact you privately to discuss payment. DaveP