Okay guys, here's what I've learned regarding P15 fuel tank sending units.
Mike, (supernice guy) at --> http://www.mykmlifestyle.com/Antique_Automotive_Parts.html
is very knowledgeable on this series of postwar Plymouth. He told me that somewhere during the production period the dash gauge on these cars was optional, due to an $8 difference in the cost of the gauge. Either a 1 wire electromagnetic gauge, or a 2 wire thermostatic gauge was used.
Therefore, a 1 wire sending unit can be used on a 2 wire gauge, as well as a 1 wire gauge, but a 1 wire sending unit won't operate entirely correctly on a 2 wire gauge. The gauge will read full on a full tank, drop off to about half, and then remain there until the tank is almost empty, then drop to empty. ie: The two wire gauges requires two hot wires to the sending unit in order to operate correctly.
Hope this helps for anyone going down this road.