[GarySP]

It appears that the two screws at the back of the chassis are a sort of 'locator' to adjust the face clearance at the front of the cabinet. Once it has been centered the screws (notice the elongated screw cut-outs) would be tightened to hold it in position. The four screws underneath would then be tightened to lock the chassis into position. The four screws underneath are very probably sheet metal threads tightened into pressed screw anchors built into the chassis sides. They mostly rely on friction fit between the threads and the screw anchors. Over the years these can sort of "weld" themselves to each other due to humidity, dirt, and corrosion. I would suggest if they are hex head to use the correct socket (preferably a six-point socket) and a breaker-bar of some sort to apply steady pressure. The screws should break free with a little effort. Once out the works should slide out from the back. Keep us posted on your progress. Take care and BE HEALTHY! Gary