I think it would be on the seller to show it really is a rare cabinet. So far I have serious doubts. The color monitor dates it beyond the original title, so I don't see it being a prototype.
Can any of the aussies around here date/place the coin door? Taito and Midway had Space Invaders, so the coin door should be common with other cabinets of theirs.
Are there other examples of a cocktail having the glass edges fully surrounded like that? For production, I don't know why they wouldn't use clips and save money on materials and welding time/expense. It almost looks like the metal covers the bolts for changing the joystick. It looks like they'd also have to notch it for the button, but it's tough to tell from the angle.
Besides, Pac-Fan is right. That's IS the title screen from Galaxian Part X. There's a pic of it on KLOV. Since the seller doesn't even know what game it is, I have serious reservations about the authenticity of the whole deal.
Good catch Pac-Fan (and welcome to the forums).