All securities, sums or values existing with the depositaries designated in Article 806 I, and which are the subject of joint or several accounts with joint and several liability shall be considered, for the purposes of levying death duties, as belonging jointly to the depositors and to the estate of each of them, in respect of a share of the assets, unless proof to the contrary is reserved for both the administration and the persons liable for payment of the duties, and resulting, in the case of the latter, either from the statements in the deposit contract or from the securities provided for in 2° of Article 773.