Searching for the phrase, 'can knocked off' yielded two more examples of usage from the 1930s:
The Snows of Kilimanjaro by Ernest Hemingway (1939)
Nick said nothing. Ad stood up.
'I'll tell you, you yellow-livered Chicago bastard. You're going to get your can knocked off. Do you see that?'
Nick stepped back. The little man came toward him slowly, stepping flat-footed forward, his left foot stepping forward, his right dragging up to it.
'Hit me,' he moved his head. 'Try and hit me.'
'I don't want to hit you.'
'You don't get out of it that way. You're going to take a beating, see? Come on and lead at me.'
Lincoln Evening Journal from Lincoln, Nebraska. May 10, 1934 p 13
'It was decided the hardest beau geste in sport comes in amateur boxing. When a young man has had his can knocked off he must shake the hand and pat the back of the lad who did it. And he must grin.'
It appears, therefore, that to get one's 'can knocked off' is equivalent to being badly beaten up.