I stumbled across this while looking for photo images of WWII radios, from 1945. Lots of interesting period stuff here.
https://books.google.com/books?id=BQKNhDRiv_kC&pg=PA79&lpg=PA79&dq=mn-31+radio+compass&source=bl&ots=VLk2nkD_aN&sig=cWUM5gOjc1inV-374ixXqwqD1Y4&hl=en&sa=X&ei=BajbVJyvE8GuyATngIKIBg&ved=0CFMQ6AEwDA#v=onepage&q=mn-31%20radio%20compass&f=false
Also found some good radio info on the PB4Y Consolidated Vultee: http://aafradio.org/sidebar/PB4Y2_Line_Maintenance_Manual.html
https://books.google.com/books?id=BQKNhDRiv_kC&pg=PA79&lpg=PA79&dq=mn-31+radio+compass&source=bl&ots=VLk2nkD_aN&sig=cWUM5gOjc1inV-374ixXqwqD1Y4&hl=en&sa=X&ei=BajbVJyvE8GuyATngIKIBg&ved=0CFMQ6AEwDA#v=onepage&q=mn-31%20radio%20compass&f=false
Also found some good radio info on the PB4Y Consolidated Vultee: http://aafradio.org/sidebar/PB4Y2_Line_Maintenance_Manual.html