Were all original Maynards built on precusion receivers?
Will

Aparrently so. The transition ones and the 1873's were made from left over frames still at Mass Arms when the Civil War contract was filled. I think every 1882 I've seen was serial numbered lower than the 1873's. They were made on guns bought back from the Gov't. Only several hundred were bought at auction, the remainder were just bought outright.
      The top of the frame was cut away and a wedge shaped piece fitted and fastened with 3 rivets, two on top and one in the breech face. The rivets are actually screwed into the frame and peened over to hold the conversion plate. 
       The 1873 and early 1882 firing pin was held in place with a cross screw on the bottom. The firing pin diameter is about .140" and the backthrust on the firing pin when it contacts this screw exerts an upward force on the conversion plate. This can loosen the plate over time. Several years ago I had a guy call me looking for an 1873 firing pin. He said "Oh yeah, I need that little plate that holds it in, too" He had been shooting Triple Seven in an 1873 and had blown the firing pin and conversion plate off the gun. I could not help him out.
      The later 1882 threaded firing pin retainer is much safer, plus the 1882 firing pin is a smaller diameter.

I've seen enough of those conversions with the firing pin thru the nipple and flash hole done the same way to believe they were factory conversions. There was an offset flat faced hammer used on these that could have only been a factory item. 
The time right after the Civil War when self contained cartridges were coming of age was a time of great experimentation. Many prototype conversions were done on lots of firearms. Mass Arms was no exception.
The 1873 was the result of experiments to settle on a standard model. The thick rim was to aid extraction with the fingers. It was not conducive to repeating guns. By 1882 it was obvious that it was obsolete and the guns needed to use a more conventional cartridge.
       I've seen and handled a prototype with the firing pin positioned lower in the frame and a different hammer. I'm sure it was factory work because of the workmanship. Probably the 1882 conversion was cheaper to do than that one. Documention is rare and those that have documents don't seem to want to share information.

Is that really a "correct" hammer for a Maynard?  I've been gazing at that rifle (I think it is the same) on GunBroker and thinking the hammer is incorrect.

Also I note that the barrel to breech gap is not anywhere near as large as it would be on an 1873.  Is this chambered for the 1882 cartridge?  or were the early (1865) conversions using a different cartridge than the 1873?

George

That gun appears to have the breech gap of the percussion guns yet it is a centerfire configuration. It does not appear to be a conversion. I would not rule it out as an experimental factory configuration. The hump on the frame is the shape of the percussion guns but it doesn't appear to have been milled for the nipple or had the nipple hole filled. The hammer appears original to the gun. The one gun similar to this one that I looked at had a different hammer with the same type of firing pin. I just really don't know anything else I can explain about it.