Thanks for the message!
The fittings you are referring to on model R2576, the 2003–2006 Nissan 350Z Replacement Radiator are the automatic transmission cooler ports. Automatic vehicles require a cooler to operate properly and they are commonly embedded into the radiator's end tank from the factory.
You are not required to do anything with them, including blocking them off if you have a manual transmission. Coolant does not flow through this portion of the radiator. The transmission fluid that would travel through there is kept 100% separate from the coolant in the radiator.
Please feel free to contact us directly if you have any additional questions!