1 Original Model 1879 Springfield 45-70 Trapdoor Rifle or Carbine 3rd Form "Buckhorn" Sight Slide Top Plate. The third form sight was approved for manufacture in November 1879. (Information from the book “Trapdoor Springfield by M.D. “Bud” Waite & B. D. Ernst 1980). Note the bottom and compare to 4th form.