Hi flangewick - Bit of digging around but the hamp is equivalent to a US skirt steak from the flank of beef - behind the ribs before the pelvis is easiest way to picture this.
Hamp and Flachette are both drawn from the same section of the cow marked "flank" on the American cow, but French and US - and UK - cuts will differ slightly from exact places on the carcass.
Hamp - or la Hampe - is from the hangar and skirt cut (hanger is from the underside of the animal just above the diaphragm).
So presume a 'Special Hamp' (in the link from shaney) is a bifteck de hampe, being a steak from the flank - and probably with a suace if the restaurant calls it a 'special' but maybe the next ABer to go to Prague will enlighten?
Things you learn, eh? Now I need to impress someone with a new found cut of beef . . .