Diana MGR/model 19 possible prototype?

Diana MGR/model 19 possible prototype? 

This is a curious air rifle, with some similarities to the 1905 patent and some to the MGR/Diana 19. It has the surface-mounted cocking lever guides and the narrow cylinder of the 1905 patent model, together with cylinder casting and barrel release lever of the model 19 and the stock/flange of the MGR. It says made in Germany on the breech, like many Diana 19-type imports.

It has a strange 'shoe' guiding the cocking lever link at one end of the barrel release lever, a bit like the one seen on this 'Ansonia'-pattern rifle:

It could be a bitsa but the current owner, an experienced long-time collector, thinks not.

Guy says of this rifle:

I partially stripped the gun and had a look at it for the current owner. I have no idea if it is a prototype or just an early gun. It was not always cocking so I replaced the trigger spring with a small coil spring I had here. The spring goes onto the trigger and locates in the stock.

The foresight post on the gun is broken off and rather than remove it, drill a hole an fit a pin, I suggested getting the sight built up and file it down to suit.
I made a new breech seal out of leather and it sits in the cylinder rather than on the barrel. I also made up a leather piston washer and found a screw to hold it on as both were a bit sorry. They looked like a home done bodge and not original.

The barrel droops a little bit but as the gun is shooting high with the damaged foresight, nothing really needs doing. Im guessing it is just wear on the catch as there is no plunger. Maybe bending the cocking arm back a little might help but the then run the risk of the piston hitting the cocking arm.

The mainspring is a flat wire one, that is not that much bigger than a Webley pistol one. It was kinked at one end, but it was decided to leave it as it was even though there was an offer or straightening and retempering.

The piston has a cut out for the trigger to locate in (Ie no piston rod). The trigger is a direct trigger , a bit like a Bsa Cadet, However, there is not a lot of meat on the holding face and there is a step down in section (weak point) so I was reluctant to try to improve the trigger other than a light polish. I suggested that one of the engineers might offer to make a trigger with more meat on it, that would enable a little reprofiling.
The gun is very small to shoulder and the trigger is heavy, but the gun is fairly accurate and fun to use.