|The MR 1965 restricted concelebration to as many ministers as could
reasonably surround the altar.
As far as I can tell it never existed, but there was a modified Missal of 1964. I have seen several Missals that were published in 1964 and would parse their variety during downtime in the religion section of Olin Library at Cornell University years back. Before I knew much about what Pius XII did to the liturgy I assumed it was generally unchanged since the Tridentine Council until Vatican II. Instead of researching the pre-Conciliar liturgy I researched the transitional rites, which were endlessly confusing. I could not for the life of me find a genuine 1964 Missal, only 1962 Missals published in 1964 with alterations. In fact the 1964/5 Missal is nothing more than a tweaked 1962 editio typica, with no major variances in the propers or ordinary of the Mass. The textual differences were restricted to vernacularization of parts of the ordo Missae, the new Communion formula, the suppression of the Johannine prologue, and the reversal of the dismissal and blessing. The vernacularization scheme might be the weakest point of the 1964 liturgy and is a sore spot for those who wish to personify that "rite" as a balance between tradition and novelty.
Episcopal conferences introduced vernacular at their own discretion, allowing them to translate more than the Congregation of Rites would on its own initiative. The American bishops, peace be upon them, translated the ordinary chants of the Mass (rendering centuries of music useless with only garbage to replace it), the dialogues, and the readings. The variable prayers people were less apt to know by heart remained in Latin. The Kyrie is recited at every Mass and could easily be learned with instruction, but a variable Latin collect could only be known to either those equipped with a Missal or training in Classical languages. One can reasonably assume that the standard parts were vernacularized first because Rome did not want to venture into the time consuming endeavor of standardizing translations across several langues before the end of the Council; they wanted to see results immediately.
Among other strange features of the 1964/5 rite are the offertory procession and the option of performing the Fore-Mass from the chair. The offertory procession must have seemed every bit like the play acting it in fact is. The 1474 Roman Curial Missal speaks of an offertory procession with no indication as to what that was. The medieval rites prescribed a procession with torches during the Gradual in which the acolyte or subdeacon would solemnly bring the gifts from their place of preparation, often an altar in another chapel, to the priests, who would bless the water and wine before the minister would repose them on the altar of sacrifice. In the late first and early second millennium Roman liturgy, lay people would present the gifts to the celebrant during Mass, but only because they had fermented the wine and baked the bread themselves and at the beckoning of the Bishop of Rome. The Byzantine Great Procession is a relic of the Hagia Sophia, where the bishop and deacons would celebrate the first half of the Divine Liturgy while the priests in the skeuphlakion prepared the bread and wine in the ceremony now known as the proskomedia; the Great Procession brought those gifts to the altar and announced the intentions associated with them. The 1964/5 procession has no foundation in liturgical history, unless one considers that at some point a sacred minister had at some prior point taken bread and wine from a table to the altar.
The celebration of the Fore-Mass at the sedilia would not be as unwelcomed as it is if not for the enormous thrones, inevitably stationed at an angle, priests make for themselves. The real trouble with this is that celebration from the chair is traditionally associated with the teaching authority of the bishop, hence why Mass at the Throne and Mass at the Faldstool are considered fuller celebrations of the Roman Mass.
The Agatha Christie Indult and the foundational documents of the Institute of Christ the King (according to one ex-priest I knew) respectively directed the 1967 and 1965 liturgies. Both were summarily ignored for variations of the 1962 and pre-Pius XII liturgies. The FSSPX used the 1964/5 caeremoniale, but entirely in Latin for the sake of international students, until the French branch settled on 1962 and the rest of the Fraternity adopted pre-Pius XII. Putatively, the monasteries of le Barroux and the Fontgombault line use 1964/5, but it seems more likely that they use 1962 without the prayers before the altar (1964/5 had them), without the last Gospel, vernacular readings, and some new prefaces (taken from Paul VI's Missal); they did not adopt the ambitious range of vernacular texts nor all the ritual changes.
Given Cardinal Sarah's interest in the Ordinariate Missal and the Pope's lack of interest in anything in the Missal, one would be hard pressed to recognize this true Mass of Vatican II as anything other than a historical road sign, a point of passing.
As an aside, something does need to be done about the abusive levels of concelebration in the Roman Church today. It is an inherently good thing, well rooted in the Eastern and Western traditions, and a sign of communion between a priest and his bishop, but I remember once speaking with a visiting Dominican friar who told me that he had to look in the Missal and read up on how to celebrate Mass for our community. Why? "I concelebrate daily Mass at the priory, so I don't do this too often."