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