Don't know the exact application [img]<<GRAEMLIN_URL>>/dunno.gif[/img], but I can tell you how to read the radiator tag.
IA and AS are left and right tank codes. IA should be stamped on one tank and AS should be stamped on the other tank, if original tanks are still present.
66 is a code that dictates the size of the finned radiator core.
RK is a code that represents the whole assembly of the radiator. IA and AS tanks with a 66 radiator core. Also 66 is the code for the radiator assembly that will be listed on the assembly build sheet for the car.
If you still can't figure out this radiator I would try the below source. Robert is very knowledgeable about Harrison radiators and does a great job restoring them.
Harrison radiator expert