So I tried to add a new P-44 note to my Zimbabwe 2nd dollar set and it said the note wasn't eligible / wasn't allowed in the slot. The slot seems set up to allow a P-44a... P-44a. I've never heard of a P-44a. I can't find any reference in any of my research / shopping / looking around to a P-44a or a P-44b, just P-44.

I'm assuming this a / no a thing is the reason why the system / slot is rejecting the note. I'm assuming / hoping this was just a mistake but it's a bit of head scratcher for me as to how it happened if so.

There IS a P-46a and P-46b and a P-48a and a P-48b. Those are the 10,000 and 100,000 dollar notes in the series, with the a and b variants differentiated based on how the digits in the denomination are grouped. The P-44, the $1,000 note in the series, to my knowledge, has no such variation. 

I've seen cases in the past with these sets where the slot just hadn't been populated with scores at all before and cases where the scores where entered for an "a" variant but not a "b" or a normal version but not a star / replacement note version (or vice versa) but I've never had a slot calling for something that didn't / doesn't appear to exist.