I posted this in another thread.
I believe April had the 1701 we knew and loved from TOS, it was decomissioned 2 years prior to ST2009 and replaced with a larger, newer Enterprise we know from the movie.
The ship design and bridge look more like 1701 from TOS.
April's Enterprise IS the same Enterprise from TOS. Look at the shape of the ship, look at the picture of the Bridge. The original launch date of the NCC-1701 Enterprise was 2245. In TOS, Kirk took command of the Enterprise in 2265, but in ST2009, he took command 2258, a whole 7 years before he was supposed to.
Now, Spock was supposed to be on Pike's crew 15 years prior to serving with Kirk (per "The Menagerie"). That means that April was supposed to have the Enterprise for 1 5-year mission, then turn it over the Pike, Pike had the Enterprise until 2265 when Kirk took over, but in the ST2009 timeline, he took it over after earlier. Due to Nero, the Original Enterprise 1701 was decommissioned into Pike's 3rd 5-year mission. So, he had only 1, and into his second he oversaw the development of the "New and Improved" Constitution Class Enterprise that is MUCH larger than the Original 1701.
Perhaps the person that would have come up with the new naming convention died or was replaced, or shifted over to another field and the A, B, C, etc convention was lost.
Look at the design of the ship and of the bridge, while not exactly like Kirk's TOS Bridge, it is a lot closer than the the 2009 movie has.