Mike Phelan:
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If glycerine was used and had a different refractive index, presumably the convexity of the front would need to be different as well.
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Well, the refractive indices probably will be different: and the effect will be to alter the focal length of the lens, of course. Therefore the 'magnification' will change: though whether by enough to be noticeable is debatable.
You can always increase the magnification by increasing the lens/set distance: my old granpappy was a woodwork whiz who soon knocked up a plywood frame to hold the lens at a fixed distance from the screen (formerly the thing dangled on the set-front by canvas straps passed over the set-top and knotted round the 'bowler-hat' extension at the rear). This had the desired effect of increasing the magnification: it also drastically reduced the viewing angle and picture brightness, so the whole family ended up huddled together in the darkness.
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... I would have thought it more likely that the yellowing is caused by sunlight on the plastic, not the liquid itself.
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And again, that's very likely the case. Remember also that these gizmos are now of the order of sixty years old or so - the plastic will almost certainly have become quite brittle due to migration of the plasticisers (possibly into the paraffin..). So handle with care: if you don't have a filling-plug already fitted, you'll need to take extra care in drilling one 'cos it will be very easy to start up cracks in the plastic which may well finish the product. Which would be a pity: I wonder how many of these articles have survived to the present day (and OT, if I may be permitted for a moment: how many of those stick-on 'convert your TV to colour' rip-offs that one saw in the adverts in those days..)