They changed that advice in later editions.This is from an F4U-5 manual.
2-82. LANDING GEAR AND DIVE BRAKE CONTROL. To minimize the possibility of
malfunctioning of the main landing gear during retraction or extension, use of the landing
gear as a dive brake is permitted only in full down and locked position except in an
emergency. The maximum speed at which the landing gear extends fully and locks is 270
knots IAS. The maximum permissible speed with the landing gear down and locked is 350
knots IAS.
You're correct about partial deployment above 260 kts, we don't have that modeled.
There wasn't a "partial" setting, it was just air pressure preventing fully down and locked.
Our model locks down at any speed up until it breaks.
The dive brake control does drop the gear in the F4U but it also extends the tail wheel.