I'll have a go.
One source is Handplane Central. I looked there and found this page
http://www.handplane.com/126/howkins-plow-plane/ but thought that the information was unusually scrappy, and remembered reading something better. Thinking it was at Alf's Cornish Workshop site, I looked there, only to find a link back to this page at Handplane Central which is much more informative -
http://www.handplane.com/52/the-howkins-plane/. That article does at least tell you that the model A is rarer.
However, the page is still a bit messed up and refers to but does not include the well-known page from the 1935 Buck and Hickman catalogue, so here's a scan of that:
It is silent on the model A, however. Tony Murland's guide does list models A to D, and says that the model A was in gunmetal, while the others were all in iron. That could be why it is rarer. Presumably it was offered before the others, some time in the 20s.
I have a copy of the instruction leaflet issued with a model C, which describes B to D, but not A.
Have you got one, or the chance to get one?
Edited to add: Definitely gunmetal. Plenty of nice close up pictures on this old ebay listing
http://www.ebay.ie/itm/141934704089 , also showing the 1913 patent date.