The AB and AC designations at the end of the part number/label indicate the process used to manufacture the module.
This should not have any effect on performance of memory modules.
However, note that if wanting to use a module with process code AB together with another module with process code AC, there is no guarantee they will be compatible with each other even though they may work fine individually. Also note that using individual modules with the same process code (ie two AB's, or two AC's), again there is no guarantee of inter-module compatibility.