My VS had a Factory fitted towbar and transmission cooler, when i bought it. I removed the towbar pretty much straight away after buying it but kept the cooler, which bypassed the one in the radiator altogether. I later found out that it is also the same brand that supply B&M because when had the motor and transmission rebuilt i wanted to get a larger cooler. i released that there the same brand beside the fittings. the new B&M one bought has 1/2 threads, and then i just needed fitting to suit the rubber hose. whereas the factory type had 5/16 barbs already on the cooler
i upgraded to the larger cooler on the VS running the same setup, and bypassed the radiator cooler(to prevent the issues someone mentioned in a above reply) just in case. i then fitted the old VS cooler which is still working fine to my VY sedan and bypassed the radiators internal transmission cooler. im will be upgrading this to the larger type cooler when the VY sedan gets its new engine and transmission.
my VY one tonner has a 2100kg towbar from factory and doesnt run a external cooler at all. When i was at the wreckers the other day no VY's that had factory towbars has any external coolers either