Jim is absolutely correct – and the page from AJTT explains it all.
The Omega ‘Goliath’ calibres used were (initially – from 1894) the 30’’’ and later the 27’’’8-day. These were produced in many forms: although in a ‘pocket watch’ appearance, they were of course too large for a pocket and were intended for display. Whereas a typical pocket watch would have a diameter of about 50 mm, the ‘basic’ Goliath had a diameter of 75 mm (about 3 inches) – and then with a spacer they were 30/40’’’ (diameter about 110 mm, about 4½ inches) and 30/60’’’ (a ‘dinner plate’ at about 140 mm, 5½ inches). Many of these were made. Later there were similar versions of the 27’’’8-day.
The Omega crystal ball (which is what the OP asks about) was produced from 1894 and carried on over about 15 years – and for
@X350 XJR they certainly DID leave the factory like this. It used the 30/40’’’ (and later the 27/40’’’). The Omega reference was 9218 – and of course they appear in the Omega Archive.
Below is an image of one of the crystal hemispheres – and the rims press onto the central 30/40’’’ case (which is shown in the page from AJTT).
As with all the early Omega productions, they were not marked Omega until later (another story!) – but these crystal balls were certainly part of the regular production line. Again as pointed out above, there are many cheap modern fakes (with dials which show a fake ‘Omega’) – the giveaway is the second hand which didn’t feature in 1894!
As shown in AJTT, there was an optional heavy support (the half moon was made in brass, the cobra version in bronze): the watch was suspended by its bow. However, as an alternative to this support, the rear (movement) crystal had a ‘slice’ on which the ball could rest (to stop it rolling) – as can be seen in my image with the ruler. The problem was that many of the watches would have rolled (off the slice!) – and with others, the rims holding the crystal hemispheres will have given way, which led to broken glass: so although many were made, there are not so many to be found nowadays (hence the post above “I’ve never seen what you are describing…”).
There are other threads which highlight the fakes. It seems Omega is not interested in chasing them:
https://omegaforums.net/threads/is-...d-aka-fake-busters.43911/page-60#post-1377336
https://omegaforums.net/threads/fake-ferrari-fake-omega.164828/#post-2249697
https://omegaforums.net/threads/french-auction-house-offering-fake-omega-crystal-ball.121801/