Ray: there are examples here of Grade 6 barrels with 4 Iron 'Oxford', 4 Iron 'Turkish Star', and this 10g with 4 Iron 'Turkish' typical of Parker D4
https://docs.google.com/Doc?id=dfg2hmx7_268gzhjpd4d
I am of the opinion that nothing was more difficult to produce that 6 Iron 'Turkish' with small symmetric scrolls, nor nothing as aesthetically pleasing.
'Rose pattern' Bernard 2 is a bit garish in comparison
but might better fit Bro. Day's personality
And yes, C & D grades also used D4. Greener was of the opinion that 6 blade crolle was "over-twisted" and British 'Best' of that era were 3 or 4 Iron.