From what I understand, there were a least a few variations of Mass extant that were > 200 years old at the time of the Council of Trent, what they were like, I have no idea.
The variations fall into two broad camps: Roman and Gallo-Roman.
The Gallo-Roman include the Ambrosian† and Mozarabic. Some differences in the prayers and overall order of mass, but unless one looks carefully, they still look much like the TLM.
The Roman variants are almost indistinguishable - the most notable is the Dominican Rite, which differs in the rubrics and in that the prayers of preparation of the gifts are said before the liturgy, rather than at the start of the canon.
For comparison, the following (copied from wikipedia) is an outline of the differences of the Ambrosian Rite:
The main differences in the Mass are:* The principal celebrant blesses all the readers, not only the deacon.
- The Gospel is followed by a short antiphon.
- The General Intercessions or “Prayers of the Faithful” immediately follow the homily
- The Rite of Peace comes at the beginning of the Liturgy of the Eucharist, before the Offertory (Presentation of the Gifts)
- The Creed follows the Offertory, before the Prayer over the Gifts
- There are some differences between the First Eucharistic Prayer of the Ambrosian Missal and the Roman Canon, the first in the Roman Missal; but its Eucharistic Prayers II, III, and IV are the same as in the Roman Rite. In addition, the Ambrosian Rite has two proper Eucharistic Prayers, used mainly on Easter and Holy Thursday.
- The priest breaks the Host and places a piece in the main chalice before the Lord’s Prayer, while an antiphon (the Confractorium) is sung or recited.
- The Agnus Dei is not said.
- Before the final blessing, the people say three times Kyrie, eleison (Lord have mercy)
- The Ambrosian Rite has its own cycle of readings at Mass
- Many of the prayers said by the priest during Mass are peculiar to the Ambrosian Rite, which has a particularly rich variety of prefaces.
The main differences in the liturgical year are:* Advent has six weeks, not four.
- Lent starts four days later than in the Roman Rite, so that there is no Ash Wednesday, and Carnival continues until “sabato grasso” (“Fat Saturday” in Italian), corresponding to Shrove Tuesday (called “mardi gras”, i.e. “Fat Tuesday”, in French) in areas where the Roman Rite is used.
- On Fridays in Lent, Mass is not celebrated and, with a few exceptions, Communion is not distributed.
- Red, not the Roman-Rite green, is the standard colour of vestments from Pentecost to the third Sunday of October, and there are other differences in liturgical colours throughout the year.
Other differences are:* The Liturgy of the Hours (Divine Office or Breviary) is different in structure and in various features.
- The liturgical rites of the Holy Week are quite different.
- The rite of funerals is different.
- Baptism of infants is done by triple immersion of the head.
- The thurible has no top cover, and is swung clockwise before the censing of a person or object.
- Ambrosian deacons wear the stole over the dalmatic and not under it.
- The Ambrosian cassock, buttoned with only five buttons below the neck, is held with a fascia at the waist, and is worn with a round white collar.
- Ambrosian chant is distinct from Gregorian chant.
I can’t vouch for the accuracy, but I’ve seen documentation of much of this list.
The Dominican is MUCH closer - differing mostly in the use of a prothesis for preparation of the chalice, and a shorter confiteor, and a couple of the priest’s prayers that were added after 1200 to the Roman missal are absent. The chant is very similar, but there are differences (but to notice them, one would need to be looking at the notation side by side, or be excruciatingly familiar with one and confronted with the other).
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† There is argument that the Ambrosian may not be Gallican in origin; it’s pretty clear that it’s got Gallican features, so whether it’s Roman-influenced Gallican or Gallican influenced Roman, it’s still a mixture of both influences.