I can't state for a fact that Fuji manufactured other brands but it would not suprise me. This was fairly common practice. Also, the Fuji S10-S from the era was called a "Special Road Racer", increasing the probability of your Executive being built by Fuji.

The JUN stem, Cherry brakes and stamped dropouts were typical fare on low end, Japanese models from the era. However, Fuji S10-S from this period typically used Dia-Compe brakes and Nitto stems, so it's not simply a rebranded S10-S. It could even be a case where Executive was a Fuji owned brand, but downscale from the Fuji brand, so as not to effect the Fuji brand reputation, much the same as Honda offering cars under both the Honda and Acura badges. It would be interesting to see some pics and make a comparison between between the two models on details such as lugs, fork crown, dropouts, etc.