There were two variations of the Colt Model 1878 hammer guns. The early version didn't have a rib extension --
The later version had a dollshead rib extension, solid head plungers, and an improved checkhook --
Those numbers in front of each description are E.C. Meachum Arms Co. stock numbers, not Colt grade designations.
Are those Twist or Laminated barrels? It appears that gun has a new stock and forearm, certainly not original Colt wood.
Here is an old Lightner Library reproduction of an 1883 vintage Colt broadside --
