I did learn a possible reason why the letter I was used as a third letter for about a decade. It was initially mostly in BABN prefixes (though the CBN prefix GOI did get issued) and it was likely in an effort to make the BABN prefixes last longer, especially since $100 bills were greatly rising in demand and BABN was printing all of the $100s until 2004 when CBN started printing them. It's likely that BABN prefixes were going through so fast, it would have likely been a few years before they reached printer letter D, which was a signal that CBN had to soon take over printing.
By the early 2000s, CBN took over using the letter I as a third letter, mostly for the $20 note. After the Journey series started, the letter I looked almost like a number 1, unlike in the latter Birds notes, though four Journey prefixes - BEI, EYI, FMI and EZI - did get issued. But after 2004, the letter I was retired as a third letter, and it may have been a sign that BABN was not doing well financially by that point, so using the letter I as a third letter was redundant since CBN had significant longevity with printer letters potentially going all the way to Z (we're a very long way from that point, and we will all be long deceased by the time the printer letters get to Z).
Letter I as a third letter was introduced in 1995 the same way my province, Newfoundland and Labrador, reinstated Q, U and Y on license plates fairly recently (those letters, along with I, were retired in 1985) due to car sales gaining momentum and the variety of prefixes going through so fast - using Q, U and Y was done to add more longevity to the license plates in an era of massive car sales, though "I" is still not used to date.
