Usually, flexes are indicated by a dagger †, but this uses a different symbol:
This symbol indicates the flex in the first part of the verse of psalms and canticles, but the asterisk serves for the meter or pause in the middle of the verse [sung] in choir and in common.
What book is that? The short lines suggest a two-column breviary, but Roman breviaries don't use the flex (before the advent of LOTH) and chant books aren't usually typeset in two columns.
The Roman antiphonal has two generously-sized columns, although the page is larger than the breviary’s. You can see that the spacing is not as tight as French typography of the time (and even now…) wound ordinarily require; anyway, between the number of pages needed and the size, you can’t insert pointing (italics take up more space) but the LU format allows this.
The Roman antiphonal has two generously-sized columns
But it uses two-column layout selectively, mainly (if not exclusively) for psalms and canticles. The example in the initial post seems to be from some sort of general rubrics.
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