I can't find any "historically correct" scopes mounted for the FR8 on an official basis. I have seen pics of an FR8 with a ZF39 optic on top, which would also be "era" correct and not an improbable combination based on the relationship between Germany and Spain prior to WWII.
But a Weaver K4 will be easier to find, although any of the steel bodied Weaver K series scopes (K3, K4, K6) would be correct for the era.
Jimro