Have you checked out the database here on ATP? There is one entry for H. dumortieri and another entry for 'Dumortieri' as a cultivar name registered in 1934.
I know sometimes registrations like the latter are selected clones of a species which are then registered, but I don't know if that's true here. The entry of the cultivar 'Dormortieri' lists three child plants and one is a cross between flava X Dumortieri registered as 'Sovereign'. If this is a species cross of some sort (I couldn't figure out 'flava'), which is what I would guess since the cross was made in 1906, then that would establish the ploidy as diploid as given for the cross and since 'Sovereign' in turn has two registered offspring listed as diploid also. Diploid would be more likely, I think, as a species. The most recent thing I saw was 1949, so everything is a few decades in the past concerning cultivars. The photos of 'Sovereign' in the database were used with permission from Manatawny Creek Farm which is still an ongoing concern per the web page for them. You might find someone there that would know more about it.
One more thing. Wasn't Maurice making a collection of species? He might be someone else who would know, in any case. My memory may be incorrect on him growing them. For something new that resembles H. dumortieri take a look at 'Woodlands Yellow Warbler' from Anna Carlson. Actually she has several along those lines and I think she is working with some more.
Too much info, I guess
:). Your plant just caught my interest on a rainy and very cool day.
