The first Skyraider version was the AD-1, later versions were the AD-2, AD-3 and AD-4.
When their nomenclature was harmonised the AD-4 became the A-1D, the AD-5 the A-1E etc.
The previous variants were not given new nomenclatures because they were no longer in service.
https://en.wikipedia.org/wiki/Douglas_A-1_Skyraider