Actually it is a factory upgrade... Like, genuine Nissan OEM accessories.
You buy the car stock and you get to chose if you want to have the rear pods.
For example the R32 and R34 GTR had "Nismo" body kits available and you bought / sourced it from Nissan / Nismo.
For the R32 GTR, there was the Nismo front bar, which had 2 air vents for the intercooler, side skirts with rear quarter Spats, then a Boot Lip and a Bonnet Lip Spoiler to complete the "Nismo" kit.
The R34 GTR had the Nismo front bar with extra vents and a more aggressive look with the Bonnet Lip Spoiler built in, a set of side skirts and rear pods to complete it.
To my knowledge for the R33 GTR, you could only get the Nismo front bar with 2 air vents, rear pods and a Bonnet Lip Spoiler like the R32 GTR.
The only side skirts available from Nissan / Nismo was the Nismo 400R body kit.
The Nismo 400R body kit was a redesigned front bar with more air vents, the side skirts and a whole new rear bar.
It gave the R33 GTR a tuff look.
With a nice ride height and offset with Nismo LMGT wheels, it held an aggressive stance!
Cool!