Suffolk Punch 43SL Throttle Lever & Oil Level?

Thanks John; great information. My mower's a 43SL with a square oil drain plug (horizontal) and an angled filler plug (plastic) both on the back. I can find no dip stick - front or back so how do I judge the correct oil level? Where should the (missing) wavy washer be located on the throttle assembly? There's a trapezoidal opening and spigot hole in the housing on the right side. Is it there? There's no sign of anything having been mounted axially & I haven't taken it to pieces and lost anything.