I've found some great information in the book "Arsenal of Freedom", page 75, from the fiscal year 1896 report. Six carbines were used to test sights, the serial numbers listed in the results table are: 11803, 11804, 15787, 15903, 15969, and 21451. When I inquired through the Springfield Research Service for information on my carbine with serial number 16720, they responded that they had no information on my particular serial number, the closest they had records for was a carbine with serial number 16968. I've read that the Springfield Armory would have stockpiles of parts, which they would draw from to assemble rifles & carbines, so that would explain how the serial numbers could jump around. The 1898 cartouche would make sense for when they upgraded it with the 1898 bolt.