Interesting that the Westfield tank fits in the Murray frame so closely. I do think your frame is earlier than 1940, more likely a 37. The truss rods are the earlier type and the frame still has the Alemite fittings which I don't think were still being used in 40 although the frame style was.
I own an Elgin with that frame that has the same truss rods and Alemite fittings. Mine has a higher S serial number and the Elgin/Musselman hub has a date code from the third quarter of 1937.