Something is definitely up with Ontario, and whatever that is, it's happened relatively recently. When I bought my RD-9 quite awhile back, it was a quarter inch of 5160 steel hardened to a Rockwell rating of 57-58, like the one example says above... I also think JG might have chimed in on a thread concerning that and other topics related to his use of 5160, if I recall correctly.
Today, if you look at the 2012 Ontario catalog, it still says a quarter inch (or .26 actually) of 5160 steel, but only claims a Rockwell hardness rating of 53-55. I don't know if that's a misprint or if it's accurate, but that seems like a rather significant difference if it's not a typo or mistake.
RD 9 - Black Micarta
Description
Specifications
National Stock Number (NSN) -
Edge Type Plain
Lock Type Fixed
Deployment Mechanism -
Pocket Clip Configurations -
Hardness 53-55 HRC
Overall Length 14.5 in (37 cm)
Closed Length -
Weight 1 lb 3.2 oz (0.54 kg)
Blade Length 9.5 in (24 cm)
Blade Material 5160 Steel
Blade Grind Flat
Blade Angle
Blade Thickness 0.26 in (0.7 cm)
Blade Color Black
Blade Finish Powder Coat
Blade Stamp RD-9 / ONTARIO
Handle Color Black
Handle Material Micarta
Handle Thickness 0.83 in (2 cm)
Sheath Type MOLLE Compatible
Sheath Color Black
Sheath Material Nylon + Kydex
Packaging Type Plastic Bag
Packaged Dimensions 17.5 in x 3.5 in x 2 in (44 cm x 9 cm x 5 cm)
Shipping Weight
Release Date -
Country of Origin United States of America
http://www.ontarioknife.com/catalog/item/199