That appears to be a stock switch. I drew the diagram below when I was troubleshooting one in 2006. The switch is shown three times. Top one is with the switch "off", ie. in the middle position. The second is with the switch signaling a right turn and the third is signaling a left turn. The black dots are the fixed contacts in the switch and the yellow pieces are the sliding contacts.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
