User blog comment:Austin J.Moulton/Need help naming 4/@comment-27281912-20161204235654/@comment-24020008-20161205001442

Well, if I recall correctly, the Type-"numbers" indicate the year it was first recorded, with the exception of things like grenades and melee weapons. So a Type-22 can't exist, as the Covenant made contact in 2525, and a Type-67 would be much further into the timeline than current Halo, almost a decade later.