Bob (and everyone) :
Thanks for having a look.
I have never had occasion to contact CMG, but they appear to act as agents for the Estate / The Mark Twain Company / or heirs and assigns in the domain of trademarks. Trademarks *can* last something like forever - e.g. Coca Cola. Some recent books - the Bible According to Mark Twain, for one (I think - you could check via Amazon Reader) - include a credit line to them.
I think the "right of publicity" is similar- as with Elvis imitators, who are licensed I think - so for folks who wish to imitiate the inimitable MT; a license seems to be available.
(Disclaimer: I only know just enough about copyright to be dangerous ;-) - but I'm not well versed in TM or the other IP rights)