A new Wagner F28851 Brake Wheel Cylinder that I got at an auction of vintage car parts.  

According to my Wagner parts catalog, this is for use on the Lower Rear Position on both the left and right wheels of:
  • DODGE TRUCKS  (1958: Series 400; LD, LP, and LS)
  • DODGE TRUCKS  (1958: Series 600; LT (6 wheel, rear tandem traction))
car auto automobile automotive classic braking system commercial delivery utility pickup pick-up 58  (CABINET K, SLOT 8)