So if I got this right ,I think,it is where they drilled the hole for the fire channel through the patent breech “chamber”to the nipple hole. Then they needed to plug the end with a,preferably, Non-removable filler plug(one less thing to worry about being tampered with) thank you that makes sense as I remember seeing pictures of the older screw in the snail under the nipple wondering about it.(I thought it was a clean out screw!) thanks fyrstyk.
In retrospect I should have looked at my .50 barrel much later serial 304xxx and it is much more indicative of a plug in the same spot.
One more question,if one wanted to remove the breech ,do these plugs intrude into the barrel / breech threading so as to lock the breech plug in place or at least make it harder to unscrew the breech?( note to nervous nelly’s I don’t at present plan to remove this plug but should I in the future desire to ,I see no big deal about it and want to have an idea of all the angles and obstacles involved,I am capable and willing to take the risk in order to have access to both ends of the pipe!) OWNERS DISCLAIMER AND ACCEPTANCE OF SUCH RISK.
Thanks again,for that answer to a long time puzzle Fyrstyk.Also thanks to you Joe ,you did have the answer.