GB,
Just a guess, but I'm thinking that if you had slugged the bore on that Marlin, you would have found they had cut a 45 Colt chamber in a barrel blank that was bored to launch .458 bullets for 45/70.
6/1000ths might not seem like a lot, but, I know from experience, that 200 grain jacketed bullets intended for 44/40 (.427) don't shoot worth a darn in a Ruger 44 carbine (.429).