I believe that these are the GRP tires that Jelayne was initially referring to:
Part Numbers (
source)
Fronts:
GVW04C 35 Shore
GVW04D 37 Shore
GVW04E 40 Shore
GVW04F 42 Shore
Rears:
GVW06C 35 Shore
GVW06D 37 Shore
GVW06E 40 Shore
GVW06F 42 Shore
And yes, these are
GRPs.