"but had a wide upswept corsair's moustache in compensation"

It is assumed LMS was implying the moustache mentioned here was typical of that worn by the swashbuckling French corsairs, although this particular corsair (pictured) is minus a moustache and others viewed boasted dissimilar styles...

However, the corsair (a glorified pirate) was someone assigned to raid enemy ships on behalf of France and their flamboyant reputation and look is probably what LMS is referring to here.