Ok on pump name the pre 2000 ABAC pumps were 2 number, B39 was replaced by the newer B3800 the other pumps gained 00 at the end. B39 was a great pump but now long gone.
The B2800 is a much smaller pump than the B3800 see below, not my image the left hand pump is a B3800B due to the aftercooler, the head and valve plate are the same on both pumps, the head is symmetrical and can be turned 180 to get the outlet the side you need.
The B2800 displacment is smaller due to a shorter piston stroke, the B2800B can have an air deliver close to the B3800 (no B) by running faster at the expense of heat, noise and pump life
The B2800 is a much smaller pump than the B3800 see below, not my image the left hand pump is a B3800B due to the aftercooler, the head and valve plate are the same on both pumps, the head is symmetrical and can be turned 180 to get the outlet the side you need.
The B2800 displacment is smaller due to a shorter piston stroke, the B2800B can have an air deliver close to the B3800 (no B) by running faster at the expense of heat, noise and pump life