There seem to be two hands at work in the margins of Western MS 97. Below is the basis of the claim: on the two images chosen to represent Hand 1 (Figures 1 and 2), I have highlighted the letters E, R, and A to show their calligraphic kinship. The hand is an informal Gothic script and therefore difficult to date based on style alone. Based on the saints added by Hand 1 (see the Discussion), it is likely that it dates from the 15th century.
Figures 3 and 4 highlight the letters O and F in two marginalia by Hand 2. Hand 2 is probably from an earlier date than Hand 1, perhaps from the mid-14th century. (Procopius and Martial, two of the three saints added by Hand 2, were incorporated into the Dominican calendar in the 1330s.)