The references are a sort of shorthand that had been used by court clerks and clerics in recording dates and are drawn in part from the latin origins of cardinal numbers, thusly:

7br = septem (in Latin, cardinal number 7) + br (to represent 'ber') = September (in its expanded form)

8br = octo (in Latin, cardinal number 8) + br = October

9br = novem (in Latin, cardinal number 9) + br = November

Xbr or 10br = decem (in Latin, cardinal number 10) + br = December

Note that this sytem of shorthand notation for dates was never used for any of the other months of the year e.g. January, February, et cetera. This is attributable to the fact that none of the other months of year represent numerals. Neither do the abbreviations ever appear without the added "br" for, even in the historical contemporary sense, the abbreviation for a month simply as the numeral "7" would not make sense and lead only to confusion.

The origin of the names of the months September through December definitely derives from the old Julian calendar (introduced by Julius Caesar) whereby the first month of the year was deemed to be March and, under that system, September was deemed to be the 7th month of the year.

I have grown so acquainted with these date abbreviations and what they signify that, now, when I encounter them I simply recite the Latin ordinal numbers to provide the name of the month i.e. septem, octo, novem, decem. No need to count the months to remember the expanded form of the abbreviation.