Hi Olivier

½" and 13mm ribbons are totally interchangeable. There is only 0.3mm (0.012") difference if there is actually a difference. The USA still works on the Imperial system, so anything close to ½" is called ½". Europe has been metric for a long time, so feet and inches are all but forgotten. If you measure the distance between the flanges on a standard 2" (51mm) ribbon spool, you'll find that it's actually 14mm for a little clearance.

It's not until you start getting into the 7/16" (11mm) or the 9/16" (14.3mm) ribbons that you may run into problems with ribbon feed or print quality. At the moment, I'm running a 9/16" ribbon in an Olympia SM-7 with italic typeface. When I load the machine with ½" ribbon some of the upper case letters get their tops cut off on the paper. Hope this helps,

Sky