So I did some more research in the catalogs. Here's what I found.

For 1969 all fullsize cars including wagons used part number 3901698.

That's true of 1970 as well except for wagons they added part number 3968773 which I guess is the wagon version.

I verified this in both the US Impala catalog and the Canada Pontiac catalog (same chassis). Both use the same wheel across the line in '69 and then introduced the special wagon version in '70.