According to IPDB, only one prototype backglass was made. It was owned by the game designer Steve Ritchie for many years, and is now signed in sharpie by him.
The glass is thinner than a regular backglass (so maybe not safety glass), and the artist Tony Ramunni's signature is different. The prototype shows a turquoise clad Knight, while on the production glass the armour is grey. The horse appears to be more blue on the prototype, and the appearance of the "Black Knight" top lettering is different, but I suspect it is due to lighting or touch-ups made to the blue which is prone to flaking.
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