Puzzling Swiss/French date abbreviation?

Question

Does anyone know what the capital 'I' might represent in this date? There is no French month that begins with 'I', and the document uses numbers for months as well as spelled out month names interchangeably. I've found every month written somewhere in the document, so I couldn't figure it out by process of elimination. The document is a pedigree chart prepared by a genealogist at the Fribourg Archives in Fribourg, Switzerland in 1936, and there are many dates where this 'I' represents a month. The pedigree chart's information seems to be derived from church records, if that's a clue.

I'm baffled! Any guesses?

Answer 1

My partner who is Swiss agrees that it is a roman numeral "I" and stands for the first month of the year, i.e., January.

Answer 2

I think if you delve deeper you may find that the "I" is the Roman numeral for 1 and represents the month of January. I think Roman numerals are being used for months to avoid confusion between date conventions that may place the day number before/after the month number.

This conjecture seems to be supported by a Writing month with roman numbers Q&A from the History Stack Exchange.

When I worked in an Australian museum the use of Roman numerals for months on specimen collection dates was a convention we used, albeit with lower case Roman numerals. For example, 4.iii.1950 is unambiguously 4 Mar 1950 whereas 4/3/50 might be 3 Apr or 4 Mar 1950, 1850, etc.