It might make sense. The letter I was used as a series letter after the version with serifs was first released later in 1994. Using it as a series letter on the Journey series with the updated font for the serial numbers was likely short-lived because the letter I on our newer banknotes post-2000 has shorter serifs and could easily be mistaken for a number 1. On the Birds series, the letter I looked more distinguishable as a letter than the number 1, and was therefore used as a series letter. Initially, only BABN prefixes used series letter I, with the exception of GOI (CBN printed) on the $5, but after the turn of the century, CBN prefixes started using series letter I until it was completely dropped in 2006 after EZI was printed). The letter I was discontinued as a series letter after the release of EZI in 2006, but the letters I and O continue being used as denominational letters on the $20 and $5 respectively.
As for the short-lived AWO and EWO, that may have been experimental, but it failed. Maybe some machinery detected AWO and EWO as a two-letter prefix followed by eight digits, implying the letter O looks almost like a number zero. AWO and EWO were released around 2002.
The letter Q was never used on our serial numbers, and never will be. It may be due to looking close to both the letter O and the number zero.
