When an older power supply had not been opened in years/decades, it sometimes takes more than just removal of the four screws.
Turn the unit upside down, insert a thin screwdriver into the gap formed between cover and base and, going around the two seems, carefully crack any seal formed by dirt or possibly leaky Stabilyt/NiCad cells.
You cannot break the gap open, as there are four lips welded to the inside of the cover that clamp down on the base of the unit, which keep the cover flush with the base. But you can move the cover slightly up by going around the perimeter of the base plate with the screwdriver and slightly leveraging the edge of the cover upwards. Then yank on the handle to lift off the cover. That should do it.