As according to Van den End and (reading between the lines) Van Dale, verantwoording can mean an account as well as accountability, my understanding of the sentence is that the securities in question are deemed to be mutually interchangeable, so the bank is not obliged to maintain an account of the numbers of the individual securities (i.e., it need only maintain an account of how many securities of a given type are held, but not the numbers of the individual securities).
There may be an English term equivalent to 'nummerverantwoording', but for that you need someone with a financial background, which I'm not.