Grainger.. darn you made me look it up
and sure enough its there in appendix 1 to FCL 2.245.
Like you said a H300 LPC will cover you in several piston helicopters except R22/44 for instance.
So back to the original question. These helicopters are treated as a class for the purpose of prifiency checks: Bell 47, Brantly B2, Hughes 269, Enstrom ENF 28, Hiller UH12.
Othervise a "class" does not really exist at all in the JAR. All helicopter are "types".
I believe that in the F/W world all turbine and jet powered airplanes are "type" and all piston singles are one "class" as well as piston twins.