Assessment and collection after limitation period
(a) The term “overpayment” includes that part of the amount of the payment of any internal revenue tax which is assessed or collected after the expiration of the period of limitation properly applicable thereto.
Rule where no tax liability
(b) An amount paid as tax shall not be considered not to constitute an overpayment solely by reason of the fact that there was no tax liability in respect of which such amount was paid.