I've seen references to 16g and I found this on eBay. A bit longer than this one, but similarhttp://www.ebay.co.uk/itm/RAF-Canberra-Hunter-jet-engine-starter-cartridge-brass-case-trench-art-No-10-Mk3-/200811414516?pt=UK_CPV_Aviation_SM&hash=item2ec14b03f4
I'm betting Solus is correct.en.wikipedia.org/wiki/Martin_B-57_CanberraThe Martin B-57 Canberra was a United States-built, twin jet engine tactical bomber and reconnaissance aircraft, which entered service with the United States Air Force (USAF) in 1953. The B-57 was initially a version of the English Electric Canberra built under licence. However, the Glenn L. Martin Company significantly modified the design and produced several unique variants.The retirement in 1983 of the B-57 ended the era of the tactical bomber that had its beginning with the World War I De Havilland DH-4
Is it part of a shell from a 37mm AA gun, either the Soviet 61-K or the German 3.7cm FlaK 18/36/37/43? Or any other 37mm?