I really don't know either, as Chuck says the picture shows a L drum. Van was outside with a Thompson in a basket as a lookout, possibly had a L drum in there. Brainerd was cold that morning, about freezing. This is why I guess they/others tended not to use a drum hidden beneath the heavy overcoats. To bulky. They were using a Buick ( stolen) as a get- a -way car, these were big cars in the day- Guys loaded with Thompsons, big heavy overcoats, $$ bags sometimes hostages . Not Terraplanes, these were the personal cars. Fast.
I can't remember but Dillinger was robbing a bank or something and his L drum dropped out of the gun, so I guess they did use drums sometimes.
The Nelson Thompsons at Little Bo had L drums as did the one Van had there, actually Dillinger's Thompson.