AUSTRIAN POSTMARK UNKNOW

Hi eper
I don't collect Romanian stamps and that's why my opinion can be corrected by experts.
Your stamp is issued by Romania (the effigy of King Carol I) and I think the postmark is Romanian.
Cernavoda is a port city on the Danube that after 1878 (along with the Dobrogea region) left the Ottoman Empire and became part of Romania.
Regarding postal traffic on the Danube - Cernavoda - Constanta (port on the Black Sea) - Constantinople, I found some information on the website "romaniastamps/Posta locala" about 2 companies that had a concession for postal transport.





In a week I will receive Kiriac Dragomir's work - "Stampilografie Postala" and I will be able to check if the cancellation is Romanian.
What catalog are you referring to when you wrote that you can't find the cancellation on your stamp?