In various books starting (I think) with the 1908 Graduale , there is described a way of singing the Gradual in which the choir sings the R, then the cantor(es) sing(s) the V, and then the whole choir repeats the whole R.
This is said to be juxta ritum responsorialem which appears in the English Liber as "as in the responsorial rite".
What is this ritus responsorialis? Do they mean the responsory at Matins, which is not much like it but a bit? Or does ritus here mean custom and usage, as we might say "in the responsorial style"? In which case the English is mistranslated.
That will be a reflection of Solesmes scholarship building on the exposition in "Les Mélodies Grégoriennes (d'après la tradition) Dom Pothier 1880". See page 440ff (in this pdf 254/288) http://www.archive.org/stream/lesmlodiesgr00poth#page/n253/mode/2up . The current Ordo Lectionem Missae uses modo responsoriali which as you say means "in the responsorial style".
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