I just measured my Vintage Koken Shell Horse Hide strop & it is 0.154" thick which is around 4mm I think. Not up on metrics really. My other Shell horse hide vintage strop from Jager Barber Supply in Springfield IL. is 0.133" thick & the rough out treated sharpening strop with it is 0.125" thick. Both of these have turned into great strops with lot's of cleaning, small amounts of Fromm strop treatment , mink oil & lot's of hand rubbing.
Dave