Thought I'd throw in my 2cents.(just a personal viewpoint)My '53 100(Baby) had a corvette engine when I bought it back in 1972, It's chassis#143703,very early car-2-piece dash, adj steering column,1-piece sidescreens&aluminum boot&bonnet. I found a 55 100M engine(#1B213811)in Seattle in '72.Put it back to semi-stock(Healey powered)?& have enjoyed it ever since,But I would never, nor want to call it a 100M.It does have a large # of orginal Le-Mans options.See my attached photo of orginal brochure.I consider Baby- Le Mans modified or optioned. Heres an except." After Healey raced the 100 at the 24 hours of Lemans the Healey company put out a kit to upgrade the performance of the Hundred. It included a better cam, high compression pistons, larger carbs and inlet manifold, a new distributor, a larger front roll bar, and a vented engine cover with leather strap. In 1955 you could buy the kit install it yourself or have the dealer install it. This made your car Lemans modified. In 1956 the Healey factory pre-made cars with the kit already installed and called them 100-M’s. After this many folks started calling any car with the lemans upgrade an M. But technically only Factory built cars could carry the M name. All other upgraded Hundreds are technically called “Lemans”." This upgrade can be made anytime in the cars life. Of course, this is just another viewpoint,cheers Genos2 P.S I've got pic of my orginal brochere on my photos