Ultradog, my 1973 S-O-S 4000 has the engine mounted piston pump. All 3 cylinder S-O-S 4000's had them. I think that the 8 speeds with independent PTO might have had them as well, but I'm not 100% positive.
My S-O-S 4000 also does not have a hole or plug in the location you indicate in your picture. According to my owners manual, my level check plug is one of the bolts on the perimeter of the dummy plate that's there where the gear pump is in your picture. It's actually a little lower down as well, as I have to pull the brake springs and move the footrest out of the way a little to check the level on mine. That's why I made a dipstick the first time I checked the level. My dipstick is a 12 inch or so long nylon wire zip tie that I put down the hydraulic fill hole. I can get an exact measurement later when I go out to my garage.
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