The 100M cars just had the same headlamps as the other BN2 series cars, because they were BN2 series cars and headlamps were not part of the list of items changed to make one into a 100M. I've heard that many/most/all of these headlamps were changed when the cars got to the USA for legal reasons or force of habit or complying with mythical regulations or something like that. (I've heard that sealed beam units were installed when the cars reached the USA, but haven't seen any documentation. Personally I am slightly skeptical that in those pre-over-regulated days that such a detail would be consistently noticed, complied with or enforced, but I wasn't there and don't know.)
I'm not a 100S expert - just like many who speak as if they are - and doubt that there was consistency in these handmade cars, produced in batches of ten over a period of several months, but the Le Mans headlamps seem to be the headlamp of choice among owners.
Sorry not to provide a definitive answer, but I try not to be too sure about things I don't know. If you're wanting to install Le Mans headlamps in your 100M or 100S, I'd say go ahead, they were a period accessory and no one should object on grounds of originality.