Second series Sport(as already stated) It 'sports' the Coltmaster rear sight that all seconds 'sport', unless fixed. Here's a 4" MT with same sight...
3rd series used fixed, Accro, or Eliason sights across the models.....
first series used fixed (with 2 kinds of fronts), 2 different dovetailed adjustable rears with 2 different apertures depending on front style. Round notch for use with bead sights, square notch for use with patridge
edit to add...
Colt also used a different, cheesy, fragile adjustable rear on its 3rd series Targetsman