http://www.bocn.co.uk/vbforum/threads/87896-ID-assistance-for-37mm-Bofors/page3?highlight=Marieberg
Quote John T. (Sweden):
"I pretty sure that MB with a crown with big cross means Ammunitionsfabriken Marieberg.
1. Marieberg had a documented production of 37mm cases.
2. Id doubt that Mariefors in Finland would have made 37mm cases in 1938.
The gun where accepted for Finnish service in October 1938.
3. the crown of Mariefors bruk do not look like the Crown on the case.
4. In Marieberg logo the M and B did "touch" each other, as on the case,
while Mariefors Bruk had a distinct space between the letters. "