I have a Girard Perregaux Gyromatic movement enclosed in a pair of lenticular glass lenses, joined by a metal band with a decorative ring with the maker's name and model. It is clearly intended as a sales aid at a jeweller so the salesman can impress the prospective customer with the movement. I haven't seen any other such demo models and was wondering just how common it might be. Sorry for the poor photo.

BTW, as it is an automatic movement, the watch will wind, run and keep time.

The only other demo watch I own is a Shell Oil pocket watch, originally filled with engine oil - until they found that they really didn't run that well full of oil. Coincidentally also made by GP.

 

I think with pocket watches they may be more common. I have this Hamilton salesman sample:

 

Interesting - and that had to be either a demo or a one off as it long predates the time when half the fancy watches out there have clear backs - even though you can never see them except when the owner takes them off and shows them to you to brag.

 

Or if you're the owner and just can't help to gaze at the beauty that is inside...

Clear backs would have been a special order/request due to the extra time and cost required to make compared to the case...though, as you say, even entry level mechanicals have glass backs now.

 

And can't afford a JLC Reverso with clear back.....

PS - back of the GP Shell Oil watch:

 

Cool looking bridges. I bet the oil did slow the balance wheel down just a little...