Who is the young man on the bicyle BB tries to borrow
after his cab is bombed? Is there a story behind his brief appearance?

I tried to post this query on IMDB and they wouldn't accept it because it wasn't at least 10 lines long!
And NO padding!!!

Dear John,

The young man on the bicycle is, as of this writing, a mystery yet to be solved. To me, he seems to have been little more than an "extra" who was thrust into the role at a moment's notice. His acting ability appears to have been practically nil, and, as I recall, required little more of him than to scratch his head at Birmingham's thought to borrow the bike for three riders.

If this person was paid anything for his appearance - and it couldn't have been much - perhaps his name would appear on some sort of record relating to the film's budget originally kept by Monogram Pictures.

Sincerely,
Rush Glick

I always imagined he was a well-liked errand boy or
messenger who happened to be passing by at the time and someone, perhaps Mr. Toler, or MM suggested putting him in an impromptu scene just for fun.

Dear John,

I think that your scenario may not be far off! Perhaps the person in question worked for Monogram in some capacity and was asked to play his small role on a whim. The scene seems to be something of a spur of the moment gag that may have been written into the script on the spot.

Sincerely,
Rush