This machine is a modest construction mech from the "Operation Rimfire" Mekton adventure. It was originally designed with a head, but that was scrapped -- a vast improvement.
The plasma cutter is for carving valuable ore out of the rock. The micromani(ulator)s are for picking up and/or operating small items better suited to the human hand. The 75mm shotgun is an add-on, in cases where this unit's base of operations (in this case, a lunar mining facility) comes under attack.
You can find a color version here: AUX-01 Otto - construction Mekton
This drawing is copyright and everything R. Talsorian Games, no infingement, etc.