Hi George,
That is most correct my friend. If you look at the "main" Release notes page, you will see the date that each version was released. The notes correspond to the actual CUCM Release and are applied accordingly;
Cisco Unified Communications Manager Version 5.1
Release Notes for Cisco Unified Communications Manager Release 5.1(3e)
21/Jan/2009
Release Notes for Cisco Unified CallManager Release 5.1(3d)
15/Oct/2008
Release Notes for Cisco Unified CallManager Release 5.1(3c)
24/Jun/2008
Release Notes for Cisco Unified Communications Manager Release 5.1(3b)
25/Sep/2008
Release Notes for Cisco Unified Communications Manager Release 5.1(3a)
25/Sep/2008
Release Notes for Cisco Unified Communications Manager Release 5.1(3)
25/Sep/2008
From this page;
http://www.cisco.com/en/US/products/sw/voicesw/ps556/prod_release_notes_list.html
Hope this helps!
Rob