It is probably, as cmac said, a case of wood shrinkage. Or, it may be that the tang has been bent while out of the stock, hard to know with handling the rifle musket itself. The P53 family of firearms have relatively soft tangs but they will not bend easily since they are short and relatively thick. Check the inlet to see if there is good contact with the inlet, I am betting that there is and it indicates wood shrinkage rather than a bend, 1/16 of an inch is not very much and is common on the rifle muskets imported for the American Civil War from the British trade.
With the exception of the fully interchangeable products of Royal Small Arms Enfield - the government factory - (none imported during the War) and the privately owned London Armoury who was making arms almost exclusively on British government contract with only small numbers imported to the South, the P53s purchased from both the London and Birmingham trades were not interchangeable and were not made to any government specifications and often with substandard parts fitted, at best, only moderately well. They were assembled quickly and occasionally poorly. Both London and Birmingham products often show signs of having been put up with improperly cured walnut and beechwood stocks. Those made with beech are often the worst and will show signs of shrinkage, especially around the barrel, lock and the buttplate and occasionally even have loose bands, especially after 150 years of sitting in a dry storage.
