r2450 = 02 & 03 WRX with the single radiator cap found on the intake manifold

r2704 = 04-07 WRX and STi with the radiator cap found on the intake manifold and the radiator (2 caps) 

Koyo is Japan's largest aftermarket radiator manufacturer. The Koyo radiator is produced from aircraft quality aluminum and Heli-arc welded. Heli-arc soldering is the highest quality standard for enforcement along aluminum connections. - Fully polished to a mirror finish. 30% more cooling than standard radiators. Perfect for Turbocharged, Supercharged & Nitrous Oxide equipped engines - Custom built OEM fit. All radiator cores and tanks are specifically designed and built for each application. No universal fit parts are used. This ensures proper fit, just like your original equipment! Made from aircraft quality aluminum and Heli-arc welded, cores are NOCOLOK-brazed

Below you will find a photo showing the radiator overflow hose routing and how it differs from 2002 to new models. You may run the newer radiator style (with added radiator cap on the radiator) with the addition of a plastic 'T' and a short run of hose.