Transnistria is a historical fiction; a region carved out of Western Ukraine by the Germans in 1941 and thrown to the Romanians as a sop for Ribbentrop’s Vienna Award of Transylvania to Hungary. All the Romanians had to do was fight alongside the Germans to get it. Although many Romanians wondered how the fight for Transnistria took them all the way to the Don River around Stalingrad.
I find no historical reference to “Transnistria” as a political entity that predates 1940.
I believe back then it was a part of “Bessarabia”, which included Moldova.