There is no such thing as an “interim Missal of 1965.”
Between the 1962 Missal and the 1970 Missal, there were a number of changes and amendments to the 1962 Missal which were released in preparation for and as part of the transition to the Mass of the 1970 Missal. These changes (e.g., permitted use of the vernacular, celebration of the Mass versus populum) came largely by the documents *Inter oecumenici *and also Tres abhinc annos. It is important to note, however, that these were not substantial changes to the 1962 Missal–that form remained pretty much the same–and so they did not constitute an editio typica of the Roman Missal requiring promulgation and all the pomp and circumstance, so to speak, which comes with an editio typica.
The Apostolic Constitution Missale Romanum, which as you mentioned promulgated the 1970 Roman Missal, states:
We wish that these Our decrees and prescriptions may be firm and effective now and in the future, notwithstanding, to the extent necessary, the apostolic constitutions and ordinances issued by Our predecessors, and other prescriptions, even those deserving particular mention and derogation.
By “other prescriptions, even those deserving particular mention and derogation,” Missale Romanum refers to all the previous liturgical forms which existed up until the point of the promulgation of the new missal. That included the modified 1962 missal.
But if that’s not enough…
Conferentiarium Episcopalium “on the obligatory nature of the Roman Missal of Paul VI” states:
When a conference of bishops decrees that the translation of the Roman Missal, or any part of it, for example, the Order of Mass, is obligatory in a region, Mass, whether in Latin or the vernacular, may be celebrated lawfully only according to the rite of the Roman Missal promulgated 3 April 1969 by authority of Pope Paul VI.
Conferentiarium Episcopalium goes on to allow for elderly and infirm priests who might find it difficult to transition fully to the Mass prescribed in the 1970 missal "to use in whole or in part, the Missale Romanum in the edito typica of 1962, as emended by the decrees of 1965 and 1967." This emended editio typica is what you refered to above as “the interim Missal of 1965.”
As to the rules issued by this Congregation in favor or sic] priests who because of their advanced years or infirmity find serious difficulties in using the new Order of Mass in the Roman Missal or the Lectionary for Mass, it is clear than an Ordinary has the power to grant them permission to use, in whole or in part, the Missale Romanum in the edito typica of 1962, as emended by the decrees of 1965 and 1967, but only for celebration without a congregation.
(Excerpts from Conferentiarium Episcopalium appear on romanrite.com/summorum.html, reprinted from Documents on the Liturgy 1963-1979.)
As you see, however, there is some fine print to this allowance. This emended missal may only be used in “celebration without a congregation.”
It is clear that the Sacred Congregation of Divine Worship did not want the emended Missal used for public liturgy and the fact that they only made the allowance for priests who were in “their advanced years” or who suffered “infirmity” (in other words, priests who would soon be dead) that they did not expect the practice to continue much longer.
We have no reason to think that we may start up the practice of using that emended missal today. In a way of speaking, the “interim Missal” died with the priests who were last permitted to use it.
That’s certainly a matter for debate, I guess.
I hope that helps answer your question!