Someone was asking about this earlier. Here's a side by side picture of standard King Size tube with 20mm filter and a European Slim with a 20mm filter. The slims are 1mm
ActiTube Filter Slim 50 pack – The Drug Store
Silverfoiltubes International Inc. - Micro Slim 5.5mm
GOLDEN CROWN KORONA Cigarette Filter Tubes SLIM MENTHOL 2x120+1x FILLING MACHINE | eBay
Ramback Ultra Slim 6.5mm Cigarette tubes
Korona Slim Filling Machine for slim filter tubes - Paperguru.de - Bu, 8,49 €