The original screw washers were cast metal with a counter sunk bolt hole for the allen screw. They also fitted over the top of the side molding to finish off the vertical line. I fashioned several out of a brass blank. This looks better that a big fender washer from the hardware store.
The lower chassis is held to the plywood base with wood screws. These screws have probably been removed and re-tightened many times over. Screw holes get enlarged and the tight fit and security of the lower chassis loosen up. This M10A's lower chassis ripped apart from its cabinet floor and smashed a bunch of stuff in the amp (during shipping). Replacing those wood screws with threaded bolts and T-nuts solves this once and for all. I used #8 fine thread bolts.
Also note that I replaced a few of the original screws that held the base to the cabinet. If you tighten the original screws and they just spin and never grab, you should replace them. You can (1) get longer screws and use the same holes, or (2) do what I did here and abandon the old holes and use new screws. I opted to use finish washers to broaden the contact area to the base. With counter-sunk screws, if you don't use finish washers, the screw can bury itself into the wood.
When the original power switch fails (and they eventually do), replacing it can be troublesome. You can use a rotary 3PDT or 4PDT switch, but you have to find once with a knurled shaft to fit the original knobs. You can also replace it with a toggle switch, but that looks a bit odd.
On this M10A, I decided to use a rectangle shaped on/off switch since the original holes in the chassis were the same shape. An advantage to this is that wall power doesn't run through the umbilical cord up to the upper chassis and back, a disadvantage is that you have to reach down the floor to turn it on and off.
Although the "I"/"O" modern switch doesn't look period, neither does the IEC C14 three-prong plug receptacle. At least it is hidden in the back.
Notice in the above picture, CE manufacturing makes the exact replacement 40x40x30x30/450V replacement cap can, the part# is C-EC4030X2-450.
The pictures below show what happens to some of these Customs. You'll go to pick it up, and the top lifts out of the amp!
You can fix this with a horizontal bracket where the center bolt attaching to the chassis's frame (as the original did), and with out-board holes drilled through the bracket and the wood cabinet. The out-board holes would have through bolts that would fasten the bracket to the wood cabinet.