Not exactly sure what you mean ... do you mean the band that you can fasten a sling fitting to? Any idea of make?
My guess is that if the band is UK or European in origin then thread will be metric, if it comes from the USA then it could be imperial thread.
Any chance you could take it to your friendly tool shop and try a bolt or even grub screw for size? My guess it 2 or 3mm.
Cheers, Phil