Ahhh ! ... another beautiful flower.
Firstly, I'm anything but an expert, however the principle of Occam's Razor says that when in doubt you are advised to go with the simplest explanation ... in this case, as expressed above, that this is not a hybrid.
If appletonianum was a less variable species than it is, then you might suggest that the leaves are shorter and broader than is typical for most ... but still within the range of plants of this species. If such extreme genetic dominance does occur between these species then it would be novel and interesting if reproducible ... experiment anyone ???
Does anyone know of hybrids between the related species bullenianum, hookerae and sangii with hennisianum. Hookerae is moderately dominant in Barbata crosses but nothing like this !!
As for genetic anomalies it is possible, although very, very improbable, that some chromosome loss has occurred in the zygote ... and/or that epigenetic mechanisms resulted in suppression of gene expression from regions of the hennis' genome that are involved in flower development. The problem with this is that you would then expect a lot of variation in the progeny ... unless ... you further proposed that due to some rare lethal incompatibility these genetic events where selected for ... but then this sort of thing hasn't been reported in paphs as far as I know ... and seed germination rates would be very, very, low ... not really believable for me.
Cheers,
Tim