Supplier advises August 2015:

Product Change Notice 
A change has been made to the housing of our 4400 series fans. 
This affects all 4400 series fans except for encapsulated 
versions (any fans with a _U_ in the part number) or fans in 
the 4400F series.  Those fans will remain unchanged as they have 
different housings than standard 4400 series. 

Reason for change 
Continuous product improvement 

Description 
Flange area modified to be completely closed

Effect 
Reduced area for ingress of dust as well as complete coverage of 
the PCB 

This is a running change, so the new and improved design will 
ship as stock of the current design depletes.  Please note that 
depending on stock levels, you may receive a mixed shipment of 
both the current and new designs. 

Please contact a Mouser Technical Sales Representative for further 
information. 

5-0815-08