As far as I know, Revox didn't produce any tape of their own and if someone knows different, please chime in here.
I would have thought that Revox rebranded tape would be BASF/Agfa/IGF but since the # is 621, and Ampex did make a 1 mil tape in the late 60's-70's that was 621, maybe it was just for North American Revox dealers (which was common practice for consumer machines dealers to carry machine branded tape to go with new machine purchases).
Is the tape dated and does it have a backing thickness printed on the box? Also, hold it up to the light,.. is it acetate?