It, however, does give the registration nr. With the registration nr. you can find the volume nr. Another electronic index is the United Nations Master Treaty Index on cd-rom, edited by Kavass. It provides the relevant information including the UNTS citation, though not the text itself, for each treaty indexed. 1-4 in vol.
Citation is therefore at the discretion of the author, provided it is used in a consistent manner.
An advantage, compared to the UNTS, is that treaties are already published after they are concluded. For the current and preceding year, for which there are not yet paper indexes available, you can use the Opmaat or overheid.nl database. Edited by E.G. Lagemans and J.B. Breukelman. This annual overview, published by the Dutch minister of Foreign Affairs at the beginning of each year, contains four lists of treaties already concluded, but not yet submitted to parliament. When these circumstances will change, the negative decision could be reconsidered.