Hi Mark
I had a 1982 SS70 that had a D stamp as well, and a Tokai specialist guy with information about it, mentioned that the D means for export to Germany.
But, I don't think that is quite right, and I am positive that something is wrong with that explanation, most likely because of the translation.
I have since seen a D stamped on some Lesters as well, and it could be that D actually means Deluxe or Domestic.
And a +1 for Gottfried, he did post that D would mostly likely mean, Domestic, I should have listened to him.
Edit - the Tokai specialist guy is from Japan.
Mick