@vroberts there was a recent discussion linked here about silicon diode vs modern replacement discs for the whistle circuit that may be interesting to you.
EDIT: My understanding of the 1.5 Ohm resistor is that when the whistle lever is advanced into the hold position, it drops the voltage from the starting position to keep the whistle motor running on PW tenders. This is incorrect as explained by John below.
As far as the number of plates, the A and D outputs each has one.
