Could this be the remains of the mythical Haenel 27 air pistol?

[Epicyclic]

Referring to the three illustrations in the 1927 catalogues. They show the model '27' with an 'open-top' trigger guard- the trigger top tight up to the cylinder (probably stopping against it ). This arrangement is also shown in the line drawing of a '27' in your book ( the one above Collectors notes ).
Being a heavily loaded area ( witness my snapped late model 26...), the trigger guard was closed at the top maybe after a re-think or testing prior to being put into production. The aesthetics are affected by the 3mm put up top but the strength is greatly improved.

Referring to the images of the straight-pinned example, my attention is drawn to the pin which appears to be a bit knocked at on the l/h/s and has an odd r/h head in the larger-seeming drilled diameter. Is this a pin that is fitted within a threaded no-keeper screw pivot drilling? Would the owner have a look at this...

I have two of these, No 2810- slotted screw with keeper and a no serial No - slotted screw without keeper for the record.