But if the article of devotion has been blessed by the Sovereign Pontiff or by any Bishop, the faithful, using it, can also gain a plenary indulgence on the feast of the Holy Apostles, Peter and Paul, provided they also make a profession of faith according to any legitimate formula.
The above grant is taken from Norm 17 of the [1967] Apostolic Constitution The Doctrine of Indulgences. Also Norm 19 of the Enchiridion [1968]
Footnote : In order to bless an article or devotion properly the priest uses the prescribed fromula, if there is any; otherwise, he makes a simple sign of the cross toward the article of devotion, laudably adding the words: "In the name of the Father, and of the Son, and of the Holy Spirit". In practice, a formula is to be used in the public blessing of scapulars, while a sign of the cross suffices for other cases.