The M 1835 was a kind of prototype of the later (also in ordnance letters so called) M 1840. Only very few M 1835 model guns were produced. They were designed after the French M 1822 Grenadeer Musket; the Ordnance bought some of these guns in France for study purpose. Production of the M 1840 (with 42in barrel = 0.68in shorter than the M 1835) was at Springfield, Pomeroy and Nippes. They were the most modern flintlock guns and regarded as 1st class when conversion to percussion began, so nearly the entire production was converted and used during the war as well as M 1816/M 1822 conversions. Today original flintlock M 1840 are much more rare than M 1816/M 1822. They were arsenal converted with the cone in barrel(=Belgian)method, some with a patent breech (Hewes & Phillips) and also rifled. There is also that story of the so called 1842 Flintlock rifled musket (but this would be an own story for a new thread. Nippes produced/converted some with a unique Maynard tape primer. With 30421 of Springfield, 2800 Nippes and 6400 Pomeroy their usage can perhaps not quite be considered as common but surely not as rare during the war.
Best regards, Marc.