This cartridge was digged in Poltava region in Ukraine. It is loaded with AP-bullet 34,29 mm length and nitrocellulose powder with square flakes. Cartridge has black primer annullus. Headstamp is H26 V. As I know this headstamp is still don’t identified in related sources.
I have a supposition that this cartridge has Austrian origin. In the mid 1920-th Austria start trials for selection service aircraft MG. Weapon in cal. 303 British also took part in this trials. In 1930-th Austria adopted 303 British cartridge for aircraft use (official designation 7,7 mm M.32 Patrone) together with MG Lewis, Vickers and Breda-SAFAT, mainly delivered from Italy.
So, I can assume that Austria made cartridges cal. 303 British for aircraft MG. Also this cartridges could be made for export in Baltic countries.
Case construction and style of headstamps indirectly confirm for benefit of this theory. The case inner construction is not typical for Britain or USA made cases.
The style of headstamp (letter H, and digits 2 and 6) is rather like to Hirtenberger patronenfabric headstamps. Of course in 1926 Austria could produce 303 cartridges at Dordrecht Patronenfabric, Netherlands, which operated as a branch of Hirtenberger in 1926-1934. I have only late style headstamp of this factory and have no idea how early headstamps looks like.
The letter V could mean Vickers, I think
Another question is a powder, not typical for Austria in those time. Austrian cartridges was loaded with Gewehrpulver M.92, which had round flakes. But before and during WWI Austria also use German Troisdorfer Plattchenpulver with square flakes for loading special purpose cartridges, because German powder had better stability and, correspondingly - better ballistic characteristics. So we could assume that for aircraft ammunition Austrian can used better German powder.
Thanks for any info or criticism regarding this cartridge and my supposition