I agree with JPR, everything I've read says two different lines but so similar you'd be hard pressed to tell them apart. Even the Super's of both make BEL's and I think pairing a Butter with a Lesser will also produce a BEL.
The lines have been so mixed I don't think it matters anymore,
unless you get one from someone who works with an original imported animal.
or can trace it back to the original line.
Yes lesser X butter is a bel.
Well they both come from different founding animals. Ralph davis produced the first lessers from an animal he called a platinum, hence the term lesser platinum. Not sure who founded the butter project, maybe someone can clarify. I do think that on the whole there are subtle differences between the two, but its pretty hard to say what your getting. A few breeders do market both lessers and butters, but they know where their founding stock came from and have kept the lines straight. You cant say that about everyone tho.