I have never seen a Merrimack Ballard, but one should be stamped that way.
I feel that you are trying to read a bit too much into that rifle, trying to fit in into some category that it probably never was in. Consider that it might just have been a war surplus rifle "sporterized" by a gunsmith to make it a more usable hunting rifle.
St. Louis was the "Gateway to the West" and as those pioneers drove under the Gateway Arch
, they often bought rifles they would need as they passed through "injun country". A number of gunsmiths flourished in St. Louis in those days; some built rifles, others sold new and surplus arms and modified the surplus guns to what they thought was needed. At a remove of 143 years, we can't know exactly and probably never will.
Jim