@Pico61, have you taken the RF block out at some point?
The user manual is not clear about this, and talks about removing the side panel, which is structural and has the transformer, oven and another unknown to me piece of hardware attached.
I have access to two screws securing it, but there seems to be two more closer to the front panel.
It is such a pity advantest gear doesn't have service manuals. The build quality of these devices is quite impressive.
Edit: photos added. The RF block from the bottom side of the instrument, and a general top view. The side panel, which is a single, machined piece, holds the RF block (leftmost), the oven, and the unknown device under it, and the transformer. When the manual states to remove the side panel, I don't think it can be referring to that.