It's really a lot trickier to determine than that, as the F4U-1A wasn't DESIGNATED as such until a little later int he war (so what is now regarded as a 1A, at the time would have still just been called a -1).
It wasn't like the 1C, which was immediately differentiated from the -1 by the addition of cannon, or the 1D by its increased ordinance payload. The 1A was more of a gradual evolution out of the -1. Somewhere along the line as the kinks were worked out and various upgrades made they decided to add the "A," back- dated to a certain block.
That being said, our 1A would have first appeared around November 1943 at the earliest, as Raptor said, because that's when Corsairs began using the water-injected version of the R-2800.
