Yeah, it's 99% likely to be women's sizing. (FWIW there are a few menswear retailers that use numbered sizing such as Lacoste for their tennis shirts and Brooks Brothers for their Black Fleece stuff).That's a good question Seb, formal fabric jackets, suits, blazers, et cetera, are always sized by the chest measurement, be that metric or imperial. The only conclusion that I can draw from the numbering sizing system is that it's based on ladies sizing. The ladies, when ordering a fitted garment, have what's known as darts in the fabric, (whispers delicately, the chesticles need to be comfortable,) and so ladies sizing tends to follow the number system, or so it seems.
You can get a much better gist of this from the Simons Leather Company website.
https://www.simonsleather.co.uk/home/sizing.php