hi,
i think you may be confused or getting mixed up. the only nismo afm's i know of are the 65mm GTR ones
your standard afm for the GTST is 80mm so you wont have a 65mm nismo one on there, that would make no sense
you probably just have a standard afm, take a photo of it please and post it up, be sure to get a photo of the label on the afm as the colour helps us identify what afm you are using. if its just the stock one (Blue or pink) then you can grab any R33 afm (matching colour) and youll be fine - no ecu or tricks needed
here is a table of the common AFM's and what they look like - which one does your car have ?