Great to see that cleaned up, it’s come up very nicely!
There are some fascinating and very novel design features in that pistol and your photos show them very clearly. I particularly like the breach design, it looks very intricate but on closer inspection probably isn’t as difficult to make as it first appears - an ingenious bit of design.

It has undoubtedly been designed and made by a very competent engineer with a good understanding of air guns and an inventive mind. The engineering is not beyond the ability of a competent amateur machinist but the design details suggests someone with a good understanding of firearm mass production. This would seem consistent with it being a prototype from one of the established manufacturers.