the bottom of the breach block either has an arch milled into it or it is straight across the bottom. 

There should be a website that shows the differencre but I can't remember the name right now. 

Once you see the difference you understand it.

I'll try to find that website.

MerwinBray

Got it, I was looking at the top not the bottom.  Both these are low arches.  I assumed since the older one was 1875 production it would have the high arch.  It's pretty rough, could have been reassembled several times in the past 135 years.  I know my dad got it 40 something years ago.

Hi Ken !!!

BTW, there is no 1875 production TD Rifle..  Get a magnifying glass and you will see it as an 1873...  Fooled many a shooter..

8>))

Ed....

    There had to be a reason why the Arsenal engineers/designers had the early and later version breech doors.  I don't believe that the arm's total weight was a valid consideration or factor.

    For you T.D. guys who are "in the know", perhaps you know why the two styles.

    I'm going to put out a guess, which is; the high arch on the underside of the door was there in order to direct escaping blow back gases upward and to the left protecting the shooter in the event of a pierce primer of seperated case.  How did I do ?       

   Creedmoormatch

I'm not a trapdoor guy, nor am I an engineer,  but I have seen a lot of bridges.  The early high arch blocks were probably made because of some theoretical strength imparted by the arch design.  Not long after, someone decided that more steel & more mass was more better than theory.

   Jon'

        Are you going to the Maryland Antique Arms Collector's show over the weekend?  It's tomorrow and Sunday, 20 and 21 March at the Maryland State Fairground in Timonium, Md.

       I'll be there tomorrow,.   .   .   . perhaps I'll run in to you !

  Webb

Sorry I didn't get back to this before now, but maybe I can answer most of the questions. First, your rifle is an 1874 or second year production. The breech blocks have been almost a study in and of themselves. Unless you're trying to put a museum quality restoration, don't get tooo excited about them There were actually seven variations of them, although the high arch is "generally" earlier. The "arch" being the major difference, as all others were marking and dimensional (slight) differences, some were water color case hardened and some just oil hardened making them black. as to why the low/high if you look at sides of the receiver with the breech block closed, there are thin "slots" between trhe receiver and block. These were the gas relief should one of the copper cases come apart which was not uncommon. Kinda why they issued "broken shell extractors with every rifle. And in closing, gor now anyway, regardless of any difference, they are ALL interchangeable. Holler if'n I missed anything. OLde age will do that to ya.

Dick

OK Ken, so when do you intend to shoot it??

I took a TD Gemmer/Hawkin I built to the range this morning. Had some good fun. Funnest was when a guy in the station next to mine said something about a "muzzle loader" being shot next to him. To be fair he had met me at an historical function where I had my M/L gunsmith shop set up and just "assumed" that's what I was shooting. Fun, none the less. Let me kknow if there's anything else I can help with. 

Dick

Sounds like a good starting load. On the light side, but it doesn't take much to kill a piece of paper. I like it.  Unless you're planning to breech seat your loads, a wad isn't  a mandatory item as long as you have compression with the bullet on the powder. As to sporterizing your TD, did you ever think of going Gemmer/Hawken style? My signature picture is one. I'm in the process of building a carving master for Pecatonica River for this stock. Hope to have it available before long. Good luck. 

Dick