I would like to contribute some important information that in/from Australia we had one of the foremost hybridisers of Philodendrons ever known at the time, ie from late 1960's to late 1970's.
His name was Leonard P. Butt (or Len as he preferred to be called) and he lived in the suburb of Indooroopilly i.e inner Brisbane, SE Queensland.
Now I was never an aroid collector, much less an enthusiast- but I do remember seeing and growing a few of Len Butt's Philodendron hybrids, housed at/in Cairns, at the Flecker Botanic Gardens, and one such is called Butt's Imperial (Philo melo-barotteanum var. selloum x P. longifolium) and another was a cross he made with Philodendron x corsinianum and another again which (I think) was called Butt's Giant Imperial which must have been a cross between P. giganteum and P. imperial-perhaps?
In Cairns there were relatively few Aroid enthusiasts- but some of these Butt Philodendron hybrids became available in the hands of local collectors and were gradually featured in local landscapes, a good example is the roundabout and main avenue leading into the Sheraton Mirage Resort in Port Douglas (where I was born and grew up) which featured heavily planted rows and walls of Len Butt's Philo hybrids- so our local (Cairns) nurseries took them on, and made them mainstream, and so at least 3 of his hybrids were propagated in a few of our main nurseries in Cairns.
One such is Equatorial Exotics, still operating and the owner is my good mate Dr Arden Dearden, so he can be contacted on firstname.lastname@example.org to contribute some photos and information about them, and I do believe that my own old mate, Don Ellison was pretty close to Len Butt- in the day, so he too would probably have some great (old) photos and cultivar names and descriptions of some of Len Butt's most important Philo hybrids too!
Also the Curator of Mount Coot-tha Botanic Gardens in Brisbane- Ross McKinnon- whom was mentored by Harold Caufield- the original founding director of Mt Coot-tha botanic gardens, so possibly Ross would have access to information or accurate records of the parentage of many of Len Butt's Philodendron hybrids because Harold Caufield (Ross' mentor and predecessor) was a very great friend of Len Butt's.
Many of these original Butt Philo hybrids are still to be found growing there in Mount Coot-tha botanic Gardens, although Ross recently retired, as of 30th of July, he can still be contacted on his e-mail address Ross.McKinnon@brisbane.qld.gov.au.
Many of Len Butt's Philodendron hybrids stayed down south, i.e not many of them "migrated" up to Cairns- where I'm from- so mostly it would be nurserymen, collectors and Aroid enthusiasts living in in South East Queensland that would have info on Len Butt's Philo hybrids or have them in their gardens still to this day!.
I hope this may help out the Aroideana people get something in writing about what we great Aussies have achieved too!