I think you are making an assumption that the trigger guard was marked at the time of conversion, and that was not likely to be the case. And also it would be unwise to assume that list of employees/inspectors/sub-inspectors was complete with dates of employment. By all evidence that was not the case. See R.N. Kennedy's excellent monograph Notes on the Model 1816 Flintlock Musket for American Society of Arms Collectors. He notes: "This list of inspectors is included only as a reference to the collector. Although I feel that the names are accurate, I do not feel that the dates of employment are correct because I have owned or seen muskets inspected by these men that were dated much later than the list indicates these men worked. Most of these dates were compiled by previous students of the arms."
Repost from me of a similar question from a year ago (or so).
Back to the original question about how the arm is marked, it essentially breaks down into three categories:
1. Initial manufacture markings by either the Armory, or private contractor including inspection and proof marks
2. Conversion (to percussion) markings, including re-proof and sometimes lock plate date changes
3. Idenification markings placed on the weapon by whoever it was issued to, or picked it up. Generally initials in the stock, etc.