Reading Look to the Lady, by Margery Allingham, I came across the apparent slang "catch a bosso," used by Lugg, the Cockney manservant, at the beginning of Chapter 6:
As soon as I caught a bosso of 'im and 'is 'arem going up that street, I come up to see what the 'ell you was up to---sir.
The meaning seems clear enough---Lugg caught a glimpse of "'im and 'is 'arem"---but I'm wondering about the etymology. Is this an instance of Cockney rhyming slang and, if so, what is the origin?
I should add that Look to the Lady is set in the late 1920s or early 1930s, so this slang might be localized in both time and place.