Where exactly are you encountering trouble with this? (I can think of several challenges)
You can store each portion of the serial number in separate fields and then define a calculation field that combines them or you can even use merge text on a layout to display the separate fields in a continuous string that looks like they are in individual fields. If you set up the calculation field I'm about to put up, just don't use it as the primary key to link to other records--use a serial number field instead and keep this field for report, display and search purposes.
"SA." & YearField & "." & DeadOfGiftField & "." & ItemNumberField
This calculation field would be set to return text.
You can also define a field that auto-enters an "SA" and use it in place of "SA" if there's any chance you might need to use different letters in the future.