"Five Little Pigs" was published around 1942-43. Agatha Christie also worked in pharmacy around that period of time, and, most likely, referenced Poisons Acts of 1933 by Royal Pharmaceutical Society of Great Britain.
It is also worth noticing that translations of the paragraph vary quite a bit:
English:
"He was very distressed by the whole thing, poor gentleman. As well he might be! Blamed himself for his drug brewing-and the coroner blamed him for it too. Coniine and AE Salts comes under Schedule I of the Poisons Acts. He came in for some pretty sharp censure. He was a friend of both parties, and it hit him very hard-besides being the kind of county gentleman who shrinks from notoriety and being in the public eye."
Russian, missing "and AE Salts":
"Он был очень расстроен случившимся, бедный джентельмен. И правильно! Винил себя за то, что приготовил эту настойку, - и коронер тоже винил его в этом. Кониум входит в список ядовитых веществ №1. Мистеру Блейку было выражено порицание в самой резкой форме. Он дружил и с мистером, и с миссис Крейл, а потому случившееся переживал особенно болезненно, не говоря уж о том, что ему, как человеку, постоянно живущему в деревне, такая популярность была совершенно ни к чему."
German, entirely missing "Coniine and AE Salts comes under Schedule I of the Poisons Acts":
"Er war höchst unglücklich darüber, der arme Mann. Er wurde von Gewissensbissen wegen seiner Giftmischerei geplagt, und der Gerichtsarzt machte ihm auch schwere Vorwürfe."
Which makes me think that "AE salts" is probably not that important, and slightly expands the definition of the poison.
I didn't find original Schedule I of the Poisons Acts 1933, though its brief overview states that it mostly consists of various alkaloids and its salts. At that period of time alkaloids were not synthesized, but obtained from the plants by extraction, so I would suggest AE stands for Alkaloid Extraction.