all royal kits typically tend to be rare, the corsair, p-51, and zero tend to be most common. I've got a pile of about 20 royal kits and 10 built models by royal.
However the Royal p-40 is the most rare, followed by the Hell Cat, and P-61 Black Widow as these are the last 3 that wee produced before they closed up. To Date, no one has shown positively identification that a "Royal P-40" actually exists.