Here is a tiny bit of info...
http://www.romaniastamps.com/sc/franceocc.htmhttps://www.hungarianstamps.com/201...f-1918-1920/The Brainard catalogue of Hungarian occupation stamps is available here:
https://www.buyhungarianstamps.com/...Ny42MC4wLjA.and maybe available to borrow from a local philatelic library?
ETA: For posterity, in case the first link goes down, here is what it says about French occupation:
Quote:
The town of Arad was occupied by the French from 30.12.1918, but with the Hungarian civilian administration preserved (the Post Offices were supplied with Hungarian stamps from Budapest until the end of March 1919, when the relationship with the new Republic was broken). The Romanian Army entered only half the city on 17.05.1919 (when a Romanian censoring office was installed near the French one). The city was taken over entirely by the Romanians only on 12.07.1919. (Dan Grecu)