Spink have the copyright as far as I'm aware. When I rewrote volume 6 of the Encyclopedia I dealt with Spink with a view to getting it formally published, but they couldn't see the commercial value in proceeding.
What a shame. And I suppose if they won't play along, you are pretty much stuck.
I assume that the secret to philatelic publishing is to have a love of the subject, and let the sales follow. Once the material is assembled, the bar to admission for publishing seems so low that you almost can't imagine why someone wouldn't proceed.
I could hope their reticence was due to an imagined total reworking of the Encyclopedia
, but of course, that isn't it.