These two photos show different waist gun configurations!
Photo number 1 shows a modified B-26B (short wing). The photo is taken on the ground through the open camera hatch (the doors of which can be seen in the bottom left and right of the frame) looking forward, likely using a ladder. This installation placed two .50 cal M2s on small platforms firing through the old side windows. A third gun was often placed in the mount forward of the floor hatch.
Photo number 2 shows the newer standard larger waist opening with sliding hatches used from the B-26B-10 through the B-26G-25. Here the .50 cal M2 is mounted on a swiveling arm. In both photos, the ammunition tracks on the sides are for the tail gun.