Chip-Maker --
To begin with the easiest question first, let me ask if this is your first Maynard and lets assume, at least for the moment, that it is an original Mass Arms Co. period correct breech piece ?  The reason for that question is connected to the manner in which Mass Arms Co. "marked" the breech piece at the factory.  The name and patent dates were about a 5 point, block font which was very shallow in depth.  There is only a 1 in 1K chance for an original Maynard to have left the factory with-out the breech piece being name/patent marked on both sides.

It would be fair to say that one of your previous owners was dealing with a serious case of surface rust and or corrosion and went about removing it in an aggressive manner, maybe even used a mechanical means such as a belt sander.  He (previous owner) would have come to the point and time to decide whether to remove the corrosion completely and lose the markings; or conversely, save the markings and live with some of the corrosion.

He (previous owner) obviously went wholeheartedly after the corrosion at the expense of the marking.

On the question of what was typical for Mass Arms to do with the percussion standing breech, there may have been more than one design changes over time.  In other words, as the re-purchased used/surplus percussion breech pieces began to arrive back at Mass Arms there must have been a "trial and error" period during which the designers explored all their possible options and alternatives for installing firing pins.

Not all of the war surplus auctioned Maynards went to Mass Arms Co.  Some smaller arms dealers were successful bidders and took theses used percussion style breach pieces into their work shops to emulate what Mass Arms was doing.  (I have a list of 2 or 3 at home and will pass than info along.)  The smaller machine shops may have had different ideas to about converting to C.F., but I doubt anyone will ever sort out which re-worker used which methodology for converting

DM
This is Maynard #3 for me, as they just seem to keep multiplying. My other two are 1873 models and the 1873 stamp is actually bolder than the patent markings on the left or Mass Arms markings on the right side. The patent and Mass Arms markings are undisturbed on this action (#10877) and there is no evidence of past refinishing.
Getting photos of the dove tailed breech piece was difficult. The black mark was needed to convince the camera to properly expose the photo, so that the dove tail line could be seen on the right.
Have you ever seen a converted Maynard handled in this way? My other Maynard's both have the breech addition held by three screw/rivets.
Do you have any information on any of the smaller arms dealers who might have been doing Maynard conversions of the percussion rifles? 
Otto  

Here's what was supposed to be the 1st picture.

Webb, the only markings missing on Otto's Maynard are the numbers "1873" and "1882". It is otherwise marked normally. I've seen guns with one or the other missing but I don't think I've seen one with both missing. Perhaps with the odd conversion it should not be considered as either.

The comment by DB, that this odd conversion shouldn't be considered either a 1873 or 1882 conversion, together with the link by DM to a type 2 Civil war era Maynard serial # 10900 caused me to a closer look at this Maynard serial # 10877.
The dovetailed insert conversion from percussion to cartridge apparently wasn't a Mass. Arms variation. 
Placing the firing pin retaining screw above the firing pin was also not standard practice and most likely lead to the damage shown in the pictures below. 
The hammer conversion was carried out by brazing a new spur into a notch but it had broken off sometime in the pass. See photos below and the hammer repair to a 1882 style hammer.
A crescent butt plate was inletted into a shortened stock. See the photos of the stock alongside an original unaltered stock and a shortened stock fitted with a standard butt plate.
The hammer cocking lever is similar to but of a different style than shown in Dr. Maynard's patent drawing of 1873 and is inserted into a hole instead of the drilled and broached hole as done by Mass. Arms.
The barrel currently attached is a 32", 40 caliber barrel in a 1882 sporting style. The corrosion prevented a chamber cast but the area between the lugs had a marking #28.
The following is what I think the preceding differences represent.
A Civil War era type 2 action, converted to a 1882 type sporting rifle by someone other than Mass. Arms. 
The action changes patented by Dr. Maynard in 1873 are present but done slightly different than done by Mass. Arms.
Does anyone know of any non Mass. Arms percussion to cartridge conversions? 

Additional pictures showing hammer damage, sights,hammer cocking lever, and the stock with brass butt plate.

DM
My mystery Maynard conversion was set up with a Maynard, 1882 style, barrel. The 32" barrel was 40 caliber, with sporting sights but the bore and chamber were too corroded to obtain a chamber cast. I suspect that it may have originally been chambered for the 40-70 Maynard.
The firing pin and spring were missing. They were retained by a cross screw, similar to  Mass. Arms conversions but the cross screw was located across the top of the firing pin, instead of the bottom. This contributed to the damage, to the rear of the firing pin area, as shown above. 
When I rebuilt the damaged area, I located the retaining screw below the firing pin. The firing pin recess was drilled similar to that done by Mass. Arms, so the new firing pin and spring follow the Mass. Arms style.
Otto