You could use a light meter to compare results. The older MKII U3's were a bit yellower than the T6's. There is no way tell the difference when purchasing. The U3's were among the last MKII's production runs.
I don't have the information, but if You have the original box of that light, it displays the version in the barcode. It is the suffix like "E". I'll try and get more substantial info to reply soon.