Turboprops operating in the North Atlantic HLA

[plans123]

Fellow Ppruners, do multiengine turboprops that are operating within the North Atlantic High Level Airspace have to report their speed as a Mach number as Jetliners do, or can they just report as a knot speed on their ATC flight plans?

I'm having trouble tracking down anything in the oceanic documentation that states the rules. Any help or information warmly received.