CathInfo.com

Source  Wikipedia:
https://en.wikipedia.org/wiki/Good_Friday_prayer_for_the_Jews#:~:text=Apparently%20this%20practice%20(of%20not%20kneeling)%20had,Jews%20were%20consigned%20to%20a%20%22moral%20ghetto%22.

"...They first consulted Dominican friar Marco Sales, considered close to Pius XI. He initially accepted that, from the point of view of faith and doctrine, there was no reason to object to the proposed liturgical reforms. However, in view of the Catholic tradition, they were considered inappropriate and not useful and argued that:^[16]

• All the criticized parts of the Jєωιѕн prayer, including the omission of the kneeling and the 'Amen', had already appeared in the ancient Church. As "venerable holy liturgy, dating back to antiquity", they escape any reformability.
• If such interference in this tradition were to be allowed to a private association, one would not come to an end and could just as well allow the removal of offensive passages in the apostolic credo, the improvers  [Impropers ?? see how the enemies flip the meaning of words? Td.] and the curse psalms from the liturgy. These contained much harsher formulations for Jews.
• "Perfidis" always means a breach of words and contracts: This is exactly what God himself accuses the Jews in the Bible. Sales referred to Deuteronomy 20:27, 31:16, Psalm 78:57, 2 Corinthians 17:15 and Acts 7:5.
• Just as God had only made a covenant with the Jєωιѕн people, only those who had broken this covenant and continued it constantly: therefore the expression "perfidis" is appropriate for them, and not for the pagans...."

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1. Impropers (The Reproaches)

• Definition: These are a series of chants or songs known as the Improperia (in Latin) or Reproaches.
• Usage: They are historically and traditionally used in the Western Church (Catholic/Anglican) during the Good Friday liturgy, specifically during the Veneration of the Cross.
• Meaning: The chants are written from the perspective of Christ, highlighting the contrast between his salvific acts for humanity (e.g., "I led you out of Egypt") and the suffering inflicted on him by the people (e.g., "you handed me over to the high priests"). It is a deeply poignant, poetic, and reflective set of hymn

Thank you for that Philip!    I hope I can find my source and substantiate my post...so, at this point we can still give them the benefit of the doubt.

That was my gut feeling, that St. Pius V didn't  forget the Triduum...Maybe the Entire Holy week was in a separate book, for practical reasons, but I am glad you found that info. Therefore that Pian missal can be celebrated by a Latin Rite priest till the end of time, as per Quo Primum, yes?
Deo gratias+


Just quickly adding this....shhhhhhhhh, it's secret!

Facebook.

https://www.facebook.com/SSPXEN/posts/a-listener-asks-father-why-the-society-of-saint-pius-x-doesnt-utilize-the-1955-h/5072008952876055/

‘In the twelve years of its existence (June 28, 1948 - July 8, 1960) the commission held eighty-two meetings and worked in absolute secrecy. So secret, in fact, was their work that the publication of the Ordo Sabbati Sancti Instaurati at the beginning of March 1951 caught even the officials of the Congregation of Rites by surprise. The commission enjoyed the full confidence of the Pope, who was kept abreast of its work by Monsignor Montini and even more, on a weekly basis, by Father Bea, confessor of Pius XII. Thanks to them, the commission was able to achieve important results even during periods when the Pope’s illness kept everyone else from approaching him.
The first fruit of the commission's work was the restoration of the Easter Vigil (1951). It was a signal that the liturgy was at last launched decisively on a pastoral course. The same reforming principles were applied in 1955 to the whole of Holy Week, and in 1960, with the Code of Rubrics, to the remainder of the liturgy.’
(Archbishop Annibale Bugnini, The Reform of the Liturgy 1948-1975, page 9)