mensis

Mensis means month. Registers use it in phrases such as die 3 mensis Maii, on the 3rd day of the month of May, or ejusdem mensis, of the same month, which points back to the month of the entry above.

Latin registers often number days from the start of the month in the ordinary way. Abbreviations such as 7ber to 10ber stand for September to December, because those month names come from the Latin numbers seven to ten.