1 Original Model 1879 Springfield 45-70 Trapdoor Rifle or Carbine 4th Form "Buckhorn" Sight Slide Top Plate. The fourth form sight was approved for manufacture in July 1880. (Information from the book “Trapdoor Springfield by M.D. “Bud” Waite & B. D. Ernst 1980). Note the bottom and compare with the 3rd form.