While I am reading a 18th c. manuscript, months of september and october are abbreviated as "7ber". I have not come across any other abbreviation for differing them. Can you please help me, is “7ber” September or October?
7ber is almost certainly just an abbreviation for September. The months from September through December come from the Latin roots for 7, 8, 9, and 10 (in the original calendar, March was the first month of the year, so September was the seventh month) with -ber appended. So this abbreviation simply replaces the Latin root with its actual number.
This is similar to using 7th as an abbreviation for seventh, except that it's based on the original derivation of the word rather than its current meaning.