There are few covers known using Task Force 6814A as a return address. Those that do were either forwarded to APO 502 in New Caledonia for cancelling or, by July 1942, were sent to APO 932 on Efate for the same purpose. Later, the original personnel of Task Force 6814 A became part of Base Command and would use APO 708 - see the second example of APO 708 correspondence below.

The next army contingent to arrive, CUB One, travelled in company with the 6th Navy Construction Battalion on USS Wharton and shared the transient code-name address of Navy UUB. The only way to distinguish whether a Navy UUB cover is from a member of CUB One or the 6th NCB is to identify to which unit the sender belonged. Because the code-name BASE BUTTON had been designated almost as soon as Task Force 6814 A arrived, covers bearing "Navy UUB" as a return address are extremely rare and can be expected to bear a postage date of 11 or 12 August 1942. Such mail was written either on board USS Wharton and held for postage on reaching Santo or were written immediately on arrival. Very few such covers exist.

Army mail using BASE BUTTON as a return address is also rare (most units on Santo at this stage were Navy and thus "BASE BUTTON" covers tend to come from Navy personnel). The creation of APO 708 on 28 September 1942 meant that all BASE BUTTON mail began to be faded out thereafter, disappearing altogether by February 1943.

APO 708 remained as the main Army Post Office on Santo until 12 August 1945. It is the commonest return address for army personnel on Santo, lending itself to many varieties and is a fertile field for research.

The only other APO to operate from Santo for any length of time was APO 27, which was attached to the 27th Infantry Battalion. That unit was at the heart of the retaking of Saipan in mid 1944. Once the island was secure, detachments of the 27th Infantry Battalion began to return to Santo for a well-earned rest. The first units arrived on 14 August 1944 and the remainder arrived progressively until January 1944. During this period, APO 27 was split, with three of its four sections on Saipan and one on Santo. Distinguishing where mail came from is difficult but, according to Jersey, mail from Saipan is roughly addressed, usually in pencil. Tidy mail can largely be presumed to have emanated from Santo.

Mention must also be made of APO 1055, which existed on Santo for a brief 46 days from 14 April 1945 to 1 June 1945, servicing the 122nd Station Hospital. There is no known mail surviving from APO 1055.