there is a smaller hole. i think about 10mm (1/8 R???). as seen on the picture, there is a brass piece stuck in there, so i thinke it has been a primer/compression tap there. the threads seems to be in inches. the hole in the small plate seems to have been for a sparkplug, but it might of course be home made. i dont know if the fixing of the springs give a clue, or if this was the normal way of doing it back then?
the cylinder is found in norway, so it might be from any nation. i have posted pictures on a similar german forum some time ago without getting any answers or sugestions at all. i have also asked some experts in belgium without any luck.
greetings Per.