My flasher has three prongs.

P = Panel = Pilot = to dash light
X or B = Battery power in to flasher (battery)
L = Load/Lamp = flasher power out to turn signal switch

http://www.ehow.com/how_7789429_test-threeprong-flasher-relay.html

I cannot find a Wagner 539 online.

Various vendors use #535 for a heavy duty 6v 3-prong flasher.

http://www.amazon.com/Bussmann-NO-535-5-2-Thermal-Flasher/dp/B000CSYL3C

http://compare.ebay.com/like/200762...ypes&var=sbar&_lwgsi=y&cbt=y

http://www.st-relay.com.tw/images/product/2007040191.jpg