I had always read & assumed that Colts machine made parts, for example the Navy revolver, were interchangeable, but quite recently heard a comment to the effect that though the parts were machine made these parts still required a skilled workman to hand finish the parts (file, polish whatever) & fit them & that they were not, strictly speaking, therefore always interchangeable. I should add that this was in reference to Colt's London factory.
 
Other contemporary arms factories such as L.A.C. & Enfield of course also polished and filed machine made parts by hand , but then gauged them carefully so that they were in fact interchangeable.
 
Can anyone comment on to what degree the London made Colt revolvers were hand fitted to  satisfy my curiosity?  Adrian.