I promised in general chat I’d post details. This was a battered specimen with the rubber grip removed so I had no reservations about dismantling it. It demonstrates the partly-exposed knock mechanism, which doesn’t especially resemble any others I’m familiar with. I took a bunch of photos to show the sequence of assembly because it didn’t seem obvious to me from the separate parts how it all fit together.
Finally, here’s a blue one in more pristine condition 
