The 85mm bore size was used in WWII Soviet artillery, tank guns and AA guns. There was probably a naval gun in that bore size as well, but that's not an area I am very familiar with.
Being a "Standard" Soviet bore size, the projectile in question could have come from ANY nation that used Soviet designs, and calibers. Which includes, directly all nations of the former Warsaw Pact, and all nations supplied by Communist China or the Soviet Union.
And it could date from slightly before WWII or any time since.
Concur on solid (AP) shot with a bursting charge cavity.
That shell could literally have been recovered anywhere east of western Europe, or anywhere in Africa or Asia.
that is IF it's a Soviet 85mm
Where did you measure the diameter??
On the driving bands? or shell body?
if on the shell body, measure on the driving bands and let us know what you get. 88mm?? 90mm??
Sometimes, like with small arms ammo, the actual diameter of the projectile is different from the name of the caliber.
Just as there are .30 caliber rifles all firing the same slug named .303Brit, 7.62x54R Russian and 7.7mm Jap, there are artillery bore sizes all firing the same actual size shells with different names. There were WWII 75mm, 76mm, and 77mm guns, US and British, all firing the same projectiles from different tanks and towed guns. Different cases, but the same projectiles.
It could be a Russian 85mm, but might not be...