The listing of Bureau Numbers for Naval aircraft at
http://www.history.navy.mil/avh-1910/APP09.PDF does not show F4U-1A, only F4U-1 (and 1C and 1D). This is from the
Appendices from United States Naval Aviation 1910-1995
However, checking actual USMC flight logs I find the use of F4U-1A quite frequent. I suspect -1C and -1D to be official USN usage, and -1A to be informal "in the field" usage, based on the Army's use of A, B, C, D, etc to indicate changes.
Enjoy!
Frank.