UPDATE: These images come from the War Wheels website. There’s a longer discussion no why changes were made to the gun placement.

Here’s what’s likely a 1941 image from warwheels.net of the Bantam BRC-40 T2 with the 37MM Gun on it mounted in the middle of the vehicle:

There were some problems with the T2, so the gun was moved to the back for version 1 of T2E1.

The army preferred this location, because a 2nd version of the T2E1 was developed that lacked a standard body (many more photos here):

Finally, there was also reportedly an anti-tank Willys MB version called the T21 with a75mm mounted in the rear, but I haven’t been able to verify it.