Ginnie Mae is adopting an alphanumeric pool number format to ensure full pool availability as numeric pool numbers diminish. Ginnie Mae intends to utilize the remainder of all outstanding numeric pool numbers then begin the alphanumeric format. It is anticipated that some alphanumeric pool numbers could be assigned as early as February or March of 2012.
For the new format, the first pool number in the new sequence will be AA0000-AZ9999, followed by BA0000, and so on. Multiple Issuer pool (MIP) numbers will be in the sequence MA0001-MZ9999. Reference
APM 10-25 for the initial notification.