I doubt if you'll find a definitive answer to this surprisingly complex issue although asking Rexel may be worth a try. Failing that if the stapler looks anything like their current range of 'tackers' it would probably be worth taking a punt on Rexel 23/6 staples.
You've probably viewed these links already but here they are anyway:
https://www.rexeleurope.com/en-gb/pr...ching/tackers/
https://www.ebay.co.uk/itm/175440231...3ABFBMuJ3J5Lxh
https://www.ebay.co.uk/itm/354401238...Bk9SR5zzlYa9YQ
Alan