As a side note, has there ever been another ship with letters at the end? Why is it only the enterprise. Its been bugging me.
Warning: This explanation is neither canon, nor is it a cannon.
The registry numbers begin with NCC because that stands for Naval Construction Contract. When a new ship is built, it may not be christened with a name until later, in which case it is issued the next NCC # in order. Debatably (and what of this isn't?), this may be straight forward numbering like 1699, 1700, 1701, etc, OR it may be that a ship type is given a prefix number (all Connies are 17) and then a particular ship receives a number in order after that (Enterprise was 17-01, Defiant was 17-64, etc). Sometimes, though, a new ship may be built specifically to put a specific name back into service. This will be be because a ship with that name served with distinction and had its name added to the List of Honor (or whatever Starfleet calls its equivalent), which means that the service will henceforth
always have a ship bearing that name. When a ship is built for that reason, the construction contract is generally a lettered addendum to the contract for the ship that earned the honor. (So NCC-1701-B, C, D, and E Enterprises, the Excalibur-A, and so on.)
Experimental ships like the first of their class will sometimes bear NX instead of NCC on their hull and in some documentation. This indicates that they are Naval eXperiments, but never actually bears on the title of the construction contract, and when that vessel is put into regular service the hull markings and such are modified to reflect the removal of eXperimental status.
I like this concept for the registries
a lot, but there are a few places that I'm aware of that it breaks down to varying degrees. I can generally No Prize those away, though. In what I consider order of credulity:
1. The Enterprises starting with TOS aren't NCC-01-A through NCC-01-F. Explanation: This is simply because Starfleet had not established its equivalent of the List of Honor - and the rules for it - when Archer's ship was in service. By the time the List existed, there was already another Enterprise (that served without much distinction) in service with a different registry. It wasn't until the 1701 that the List existed AND an Enterprise earned its way onto it.
2. The Sao Paulo did not keep its original NCC (which normally it should have since it was built under it, not
as a replacement for Defiant), and also did not take Defiant's NCC with a letter addendum. Explanation: The Federation was at war, and the prominent role that the Defiant had played in that war thus far led Starfleet to decide to disguise to whatever extent it was possible that the Sao Paulo was a different ship
at all, for morale and intimidation purposes.
and 3. The Ti-Ho/Atlantis/Yorktown/whatever-the-heck-it-was-called did not keep its original NCC even though its hull had not been laid to replace the 1701. BUT - maybe construction was not
complete (as seems to be supported by the sorry state she was in in ST:V), and upon rechristening the remainder of construction was done under an A addendum to the 1701 contract.