This is a postcard from world war one, The card is part of a series of Military slang. I understand Sperrfeuer as " Barrage ", can someone tell me what Jm means. I have tried dictionary's and on line without success.
(Extended earlier comment)
I guess, it should read:
instead, which would make the asked Jm the standard preposition im, cf. DWDS, meaning 2.
As you can see here (Book title Im Völkerringen, printed 1915), I and J are quite difficult to distinguish at that time. This already applies to print, but handwriting is even more difficult.