Well, some singers follow the "walks the streets" verse with
See him ridng in his carriage,
Home returning from the hunt;
He's got [... forgotten this line ...]
She's got corns upon her feet!

... so the "air balloons" verse could have found its way in at some time, and gotten vulgarised.

Dylan Thomas used to make up scurrilous and lewd poems to amuse his mates in the pub. I wonder if Bennett did the same? Being ex-army, he'd be familar with the genre, and it's hard to believe he didn't do gentlemen-only performances in private. But then he was never smutty in his public performances, as far as I know; using clever word-play and sheer preposterous Goon-like imagery to get laughs.

Steve