No one seems to know for sure - I've emailed Tokai JP and a noted expert in Japan - got no reply from either.
I don't buy the neck profile idea - doesn't make sense to me, and especially as we now have an 'M' it seems.
I have an '81 LS100 with a 'D', I thought it may be a show/exhibition model (demonstrator) as i only know of 2 others.
Now we have a 'V' and 'M' - OK here's another theory - direct factory ordered models by either a dealer or valued collector/customer who warranted having a stamp denoting their name - I'm thinking of the 'Leo' ordered Gibson Les Pauls with 'L' serial numbers, back in the early 80s I think, well you get the idea?
Until someone can come up with a true explanation, we'll have to keep speculating . . .