Gerald Carr (I think) at the controls of the Automatically Stabilized Maneuvering Unit, the predecessor to the Manned Maneuvering Unit that you all know and love. This model used cold pressurized nitrogen as its means of propulsion, allowing it to be tested without a spacesuit as seen here. Although it was capable of being operated on an EVA, it was not for two reasons. First, the astronauts of Skylab used umbilicals instead of self-contained oxygen supplies on EVA, and using this vehicle could result in tangled or stretched lines. Second, the risk of damaging Skylab’s delicate and much-needed solar panels with an errant burst of nitrogen was deemed too high. This is a shame; much like the USAF Astronaut Maneuvering Unit of Gemini 9, we have come so very close to operating self-contained astronaut propulsion systems several times before the MMU.
These guys are lucky to be able to test this beast in the massive space that was the Skylab’s forward compartment, though. Try flying that in the ISS and all you’d get out of it would be nitrogen-covered electronics and a bruised forehead.