These conversion drop in barrels were made in the 50s and 60s. You have to remember that at that time - a 45/70 was just an "old gun" - there wasn't the interest in them that there is today. I can remember that at that time - you could buy them for $25 - $45. You could buy a "mint" 1861 Springfield from Westchester Trading in New York for $125.00 - a shooter in good condition for $50.00. Of course the pay back then isn't what it is today either!
There was a resurgence in interest in muzzleloading - it was the centennial of the Civil War, etc. You could buy the drop in barrels through different gun magazines, etc. and they came with an 1863/64 hammer (an original one at that). It was an easy way to get yourself a decent muzzle-loader to shoot plus all you had to do was change the hammer back and put in the 45/70 barrel and you could shoot that if you wanted to. Like has been said though, those cartridges weren't cheap. Those were also the days when you could buy a H & A underhammer through such publications as Popular Mechanics for, if I remember correctly, around $19.99 for the very basic one. It came with a nice "candy stripe" ramrod as well. You could upgrade it and have a wood forestock put on and a extension for the back of the trigger guard (which was also the mainspring) with a "curly cue" on it. Turner Kirkland was expanding his Dixie Gun Works and the interest in muzzleloading was growing by leaps and bounds. Up until then, there wasn't a whole lot available in "reproductions". My first muzzleloader that I owned was a Remington Zouave - don't know who made it - that my Dad helped me buy and I worked it off on the farm. The first muzzleloader that I ever shot though, was an original 1855 model and an original Mississippi Rifle - there wasn't much else around that was "affordable".