Abilene is correct. The hollow magazine End Cap and the special double taper magazine Spring are Proprietary Cimarron parts supplied specifically for the 16 inch barrel Trappers Cimarron carries.
When I was building my 1866 and 1873 Trappers, Cimarron would not sell those parts no matter how hard I tried. If you claimed something broken, Cimarron wanted the Gun to repair. I still built my Trappers, however since I am shooting .45s, I elected to use the modified Carrier Blocks from Adirondack Jack. Those carriers are now available from The Smith Shop. Used my own Proprietary cartridge, The 45 Squirt. Made from 45 Schofield. Switched to Cowboy 45 Special to save the labor of making my own short cases to get 10 in my Trappers. Including my 16 inch barrel Henry Trapper.