Standardised "military " Thread systems:
Springfield armoury ( est. 1795) used the French metric system for the manufacture of the M1795 Springfield Flintlock Musket, as it was a derivation of the M1766- Charleville Musket of the Revolutionary War, as Modified by the French Committee of Public Safety, St. Denis Armory, in 1791-92, to use a metric system of threads, based on the earlier french system of "numbered Gun Threads" used by the Factories of Charleville, St. Etienne, etc.
One of the first actions of the Public Safety was to enforce the use of the metric system , and the unification of Gunmaking throughout France, to simplify Gun manufacture for the new Republic. BY 1810, they had a centrally controlled system of QA throughout all the Imperial manufactories, with reasonable interchangeability of most parts from all sources, and a "guide book" for Inspectors on how to carry out these QA inspections of Contractor's work ( M.Dale, M.re Imp. de Liege, 1810, republished by Liege Metropolitan Arms Museum, 1980s).
The Springfield Armory thread system was rendered into "inches" ( Thousandths of an inch)...and this was used by Prof. Sellars of Philadelphia to construct his Unified System Threads, presented to a conference of Engineers during the American Civil War..( all inch dimensions and pitches, but with the metric 60 degree thread form).
The "Springfield metric" system was used up until the adoption the the Krag rifle in 1892, which was designed and made according to the Sellars Thread scheme. ( Thread diameters, Pitches, sizes.)...so one will find some of the M1873/884 45/70 guns will use some truly metric screws, in exchange for the Springfield ones.
Given the sequential commonality of all the US Thread systems in Gunmaking for Military use, it is no great surprise that some Modern screws will interchange with the more "ancient" ones.
Regards,
Doc AV
AV Ballistics Film Ordnance Services,
Brisbane
Australia.