try this
http://www.cockpitinstrumente.de/ This may be in the german homepage so clik on the British flag and it should go to an english homepage.then clik on the Archives then Various Documents then W for weapons then go down to Board cannons of the Luftwaffe. the37mm cannon should be the 1st one up.Take a look at the machine guns too, lots of good stuff on the MG81Z. This site can be confusing so you may want to look at anything and everything and see what turns up.
Paul S.