One more - the US has the ability to allot AA - AL prefixes. You'll see some of these as DoD callsigns, and some as amateur callsigns, usually assigned to Extras.
Yes, there was rhyme and reason to the original assignment. If you called Gettysburg and asked the current people working there for historic reference, they would not be able to tell you, nor might they even care. The number indicated where you were (or more appropriately, which FCC field office you took your test at), when you moved, you needed to change your callsign, although back then you could have had a second callsign for your vacation house elsewhere.
The prefix was a rough indicator of either where you were (H for Hawaii, L for Alaska, P for Puerto Rico) or what you were (N for Novice). When one upgraded, the N was either dropped or changed to an A or B. There were other calls. A "WR" prefix was a repeater callsign throughout the early to mid 70s. A "WC" prefix was a Civil Defense club station, where the trustee could have simply had a "Push to Talk License" (a Restricted Radio Telephone Operator's Permit) at one time. Those have been gone for a very long time. In the transition between manually issuing callsigns through cards in a file cabinet (more or less) to a computer records system, the FCC also issued WDxAyy callsigns, circa 1976. To my recollection, this never progressed beyond 676 licensees in any particular number district, and I don't recall any WDxByy callsigns ever issued sequentially. A N and KA callsigns appeared in 1978 around the same time WR and WD callsigns disappeared. The arrangement used to tell you what class a station was - for example, someone coming in as a Novice got a 2x3, a Technician or General a 1x3, an Advanced a 2x2 starting with the letter K, an Extra a 1x2 or 2x1 (or a 2x2 starting with the letter A).
That's changed as rules changed. No more secondary callsigns, although there are some people who hoard club callsigns. People also license up with vanity callsigns for WX, WC, WD, or other unique prefixes that no longer have the significance they had 50 years ago. Some of the "pools" have dried up. When that happened, the default went down to the next lower grouping. People don't have to change their calls anymore when they move. So, for example, I am a 2x3 and never changed calls when I upgraded to General, Advanced, and Extra. I have a "2" callsign because I lived near New York City when I was first licensed. I've lived in 3 other numbered districts in the last 36 years and never changed my call (because I rather like it).
BTW, public safety callsigns had categorical significance, too.