Biston strataria and B. achyra are very hard to distinguish from external features. There will be more information in GME6. It looks like you need a picture of the upperside of the hindwings. And because B. achyra is known from Ukraine, Greece (at least two places, already published) and Croatia (not published, as far as I know), Serbia of course would be a possible area for achyra. So: strataria like specimens from the Balkans should be collected and the spread ones should be carefully determined (dissection as well). Sorry, from my point of view you cannot easily call your specimen strataria.
On Samos both species are flying at the same places, same time.
Best regards Dieter