It appears to me that you have a very early Mate (1939) from the ID plate. I can't see if the numbers match the freeze plug which would be good if they did. For some reason, the original tank (same for Cubs and Mates) may have been replaced with a better condition tank from an original appearing Cub, decals and all. The side covers from your Mate could have been removed and lost or sold some time earlier. I hope you can find another pair so you will be able to complete your Mate.