What’s this symbol in Europe that looks a bit like an upside-down V, or ⩘?

It's just the number "one". It's not a French specificity, in continental Europe (and in most countries with (Continental) European influence) we (almost) never handwrite the number "1" as "I" like english-speaking people do. Also, we usually handwrite 7 with a middle bar so there is no mistaking possible.

Because the writing of numbers varies between anglophone and continental countries, I have written down the different variation of numbers I myself encountered in Europe (The third 6 is a write error):

As you can see, most of the problems arise with 1, 4, 6, 7 and 9.

The first column is how I personally write the numbers (German).

Both 1 and 4 can have a underscore.
As you also see, some numbers like 1, 2 and 4 have sweeps if you write with pen.
Both 6 and 9 can end vertically, with can cause the 6 look like b.
7 has always a horizontal bar.
The third 9 has a vertical extension above on the right side of the circle, I hope it is prominent enough.

If someone has also a notable variation, I can add it later to the paper.

This PDF looks to me like standard French school-writing.

Here's an copy-and-paste of what it says the hand-written numbers should look like:

So you're seeing a (quite well-formed) 1 -- the photograph in the question shows the ascender on the left (correctly) starting about half-way-up.

... which I find easy to recognise as a 1 (and a good argument for requiring a cross-bar on the 7, to disambiguate it).

I don't know what character you used to represent that in type, i.e. "?" ... I didnt recognise that as a 1 because the ascender starts at the bottom of the line.