the boiler tank lower sight glass fitting is a small fitting at bottom of boiler. it's small enough that the heavy strain of compression nut 537-007-300 removal may crack off fitting or damage site glass 439-029-000
in a previous video I demonstrated using a Right Angle Adjustable wrench. This is an alternative method with a 2" C-clamp. Side the clamp onto tank fitting. keep clamp snug but not completly tightened down. Move clamp directly below fitting. thus wedging the clamp between frame rail and tank. lock the C-clamp in this position. clamp should be firm in place with no downward movement when forced down (mimic the downward force of 20mm wrench.) Now the 20 mm wrench can be used to remove compression nut.
Note, reassembly also requires c-clamp but you must hold the clamp with hand to prevent (upward) twisting. Use the Right angle wrenxh or 3" C-clamp by holding it sideways (off the frame at 90 degrees left side.) use both hands to grab both clamp and wrench to minimize (upward) twisting of fitting. Heave compression is not needed as over tightening may crack nut.