“For many years it was considered that Thasian imitations were a product of the Thracian tribes… In my opinion, the Thasian imitations coinage and its use are closely associated with a population who arrived and settled later within the Balkan territory. In fact, the east Celts had played a signiﬁcant role in the regional history since the 270s BC. There are good reasons to believe that the imitations of Thasos tetradrachms had an ‘international’ nature and featured interactions and activities of a culture dominated by the east Celts”…

FULL ARTICLE by Dr. Ilya Pokopov, President of the Bulgarian Museum Association and Bulgarian Numismatic Association:

https://www.academia.edu/1287658/The_imitations_of_Late_Thasian_Tetradrachms_Chronology_classification_and_dating