That vir eruditissimus, Rubricarius, has enlightened my ignorance on the significance of the abbreviation 'com. sol.' in the kalendar of the Nidaros Breviary. Following the votive office of our Lady, and preceding the sanctoral, the breviary devotes several pages to what it calls the commemoratio solita (customary commemoration) or suffragia.
This consists of two series (one for Lauds, one for Vespers) of commemorations, each commemoration consisting of an antiphon, a versicle and response, and a collect. The commemorations are made in honour of (1) the holy cross; (2) the blessed Virgin; (3) the holy angels; (4) St John the Baptist; (5) the holy apostles; (6) St Olaf; (7) the holy relics present in the church; (8) all the saints; and (9) for peace.
Rubricarius further points out that the rubrics within the body of the sanctoral direct the suffrages to be said more frequently than the kalendar entries would suggest. I'm presently following the kalendar for my weekly 'ordo', but it looks as though this might give incomplete information. The 'corrector' in an early printing house - the forerunner of the modern proofreader - was often highly skilled but overworked, so a good number of discrepancies and errors are only to be expected. As time goes by I shall collate the kalendar information with what can be found in the body of the breviary, as well as with the Missal and Ordo of Nidaros.