Album recorded and mixed in the analog domain - AAD.
A=Analog. D=digital.
The first letter stands for how the music was recorded. The second letter for how it was mixed. The third letter stands for the format (all CD's will have D as the last letter).

In late 1968, Deep Purple had embarked on a successful first US tour to promote their second album The Book of Taliesyn, and returned home on 3 January 1969. The band was considered an underground act in the United Kingdom, but word of their success in America had influenced their reputation at home, as they gradually rose in popularity and request. However, their releases had yet to make an impact in the UK, where their second single, a cover of Neil Diamond's "Kentucky Woman", had not charted and was retired after six weeks, after having peaked at No. 38 in the United States and No. 21 in Canada.
Deep Purple's American label Tetragrammaton Records pressured the band to make a single to match the success of their hit "Hush", and the band had tried to satisfy that request while still in the US for the last dates of their tour; they recorded some covers in a New York studio in December 1968, without worthwhile results. The musicians had come up with much more complex original material for their second album, and making a song that would easily fit the three-minute range was apparently becoming difficult.
A few days after their US tour, Deep Purple settled in with their usual producer Derek Lawrence at De Lane Lea Studios in Kingsway, London, already used for The Book of Taliesyn sessions, to compose and record new songs and solve the new single problem. The song "Emmaretta" (named after the musical Hair cast member Emmaretta Marks, whom singer Rod Evans had met in the US) was composed for that purpose and recorded on 7 January 1969, after four takes. The heavier and more experimental song "The Bird Has Flown" was arranged and recorded later on the same day and was chosen as the B-side for the US release. The instrumental "Wring That Neck" from their previous album was the B-side of the British edition of "Emmaretta", which was issued in February 1969 and promoted by Deep Purple in their first full UK tour. This was the first time that a Deep Purple release appeared in the UK before the US. The tour started in Birmingham on 6 February with a concert broadcast by BBC Radio 1, and went on as a series of one-nighters in clubs and colleges across the country during February and March. Deep Purple's greater visibility and their declared interest in the British public induced local music magazines to print a few articles on them. However, after keyboard player and spokesperson Jon Lord publicly stated that they would play large arenas for ?2500 per show in America then come home and only be able to book small venues paying ?150 a night, typical headlines were "Purple won't starve for an ideal" and "They lose ?2350 a night working in Britain".
(en.wikipedia.org/wiki/Deep_Purple_(album))