Pony Express,
Following is some additional info to what others have posted, I thought it might fill in some blanks. A great reference book to consider getting is American Military and Naval Belts: 1812-1902 by R. Stephen Dorsey (Collector’s Library, 2002). This is an excellent reference for the entire military cartridge belt time frame and it details quite a lot on the looped cartridge belt era.
Before the government issued a cartridge belt many soldiers altered existing belts or made/had made looped cartridge belts (pages 217-225), this would have been for the .50-70 and .45-70 and arguably other cartridges used by the army on the Frontier after the Civil War [and per D. McChristian’s US Army in the West: 1870-1880 on page 35 ‘thimble’/looped belts made by soldiers were present on the frontier at least as early as 1867]. Then came the first issue infantry looped belt - the 1876 cartridge belt (page 227-255) for the .45-70. Regarding repros of this belt, I’ve seen David Carrico’s work and can only assume his 1876 belt would be an excellent reproduction to buy were you interested in getting one.
The Mills belt, 1880-1902, existed in a variety of types. The ‘stone’ or ‘dark’ gray colored single-row cartridge looped belt of the early 1880s (page 256) would be the type to focus on were you to go with an early woven Mills type. I think current woven repros are only offered in khaki or dark blue, however a khaki one might successfully be dyed gray to get closer to the right color. Two-piece belt plates for an early belt would have been a bit different than later types, these being the regulation 1886 plate or the 1887 ‘Whittemore’ plate, the latter referred to as ‘Militia’ plates and made under Mills' control (page 277). Of interest, a couple examples of later era Mills blue single-looped infantry belts shown in Dorsey’s book have 1886 US (page 281) and 1887 NGP plates (page 279). According to Dorsey’s info the double-looped Mills belts have provenance to the 1890s and are commensurate with use in the 1890s/Span-Am period. I couldn’t locate any blue belts in his book with the earlier 1880 and 1882 two-piece belt plates though I might have missed something as I did only a cursory review of the material.
As Dryock points out S&S Firearms carries a couple woven Mills belts variants in blue & khaki, both 50 and 100 round types and a bunch of repro belt plates in their 52nd Anniversary catalog (2009). I haven’t seen these belts personally but they likely would be a good start if you are keen on a Mills belt.
I recommend you acquire the Dorsey book or other comparable reference material to get the most detailed historical info you can and compare that against available reproductions before buying a belt, whether a pre-1876 soldier-modified 'thimble' belt, an 1876 cartridge belt variant, or a Mills belt variant. Good luck in your search.
Brass