I am grateful to Jurgen Goering for his information and advice on
the recessed head screws -- at least I know what to call them now!
To remove them (which was extremely difficult) I modified a hexagonal
Allen key by filing down the faces so as to form a perfectly square
fit.  It was the best that I could do, lacking the proper tool.

Where might I get such an animal?  The screws are unknown in this
country as far as I can determine, and I had to replace them with
standard (but perfectly adequate) slot headed equivalents.

The action may well have been of Canadian origin, as Jurgen suggests.

Roger Waring