Once again JB, you are correct.
In the 1937 and 1954 series, the top letter is the series letter, and the bottom letter is the denominational letter.
In the multicoloured series, the first letter is the denominational letter, and the second letter is the series letter.
The three-letter prefixes prepend a printer letter in front of the denominational letter, but otherwise the system is identical.
So... Obviously, A/A ~= AA ~= AAA. But, A/J ~= JA ~= AJA. (~= means is comparable to, since technically they are different series of notes entirely).
Similarly, A/X (1937: did not exist, 1954: Beattie/Rasminsky $5) is comparable to XA (1972 Lawson/Bouey $5). The denominational letter X was then retired to later be reused as a test note indicator.
Here's PA:
1937: (A/P) Did not exist
1954: (A/P) Beattie/Rasminsky $1
1973: (PA) Lawson/Bouey $1
EPA: 1986 Birds Crow/Bouey $5
FPA: 1986 Birds Thiessen/Crow $5
GPA: 1986 Birds Bonin/Thiessen $5
HPA: 2002 Journey Jenkins/Dodge $5
Aside: APA will theorhetically be a future 2006 Journey $5 prefix.
The Bank of Canada's
system of prefixes is a very meticulous thing, and once you take some time to wrap your head around it, it's pretty easy to know it inside out. Trivia like the association between A/P and PA come as second nature.