The ASCC is now long gone and new reporting designators abandoned. There were many ,sometimes amusing, anomalies over the years.
The Il -20 COOT A et. seq. nicknames (they weren't "code names") were in line with An-12 CUBs B,C and D which were dedicated recce/ELINT versions of the basic transport , indeed there were some similar versions of the Il-14 CRATE as late as the mid 70's.
The Il-38 MAY was a pretty major rework of the basic Il-18 airframe with a lot of fundamental external visual changes and so justified the new designator.
All history now.