I am putting an oil pressure gauge in my 56 F100.  Midfifty's makes a gauge and sender.  I was looking at the factory shop manual and the larger truck use a sending unit that is of similar shape.  In the shop manual they make a point of telling you that the sender must be installed with the "up" marking withing a few degrees back and forth of top center.  I notice that there are no such markings on the sender that came with my gauge.  Does anyone know why this was important on the original oil pressure sending units used with gauges back in the 50"S?