Stereoisomers of Hexachlorocyclohexane
I didn’t find anything definite. However, I found a PDF from the State’s Institution for Protection of the Enviroment in Baden-Württemberg (Landesanstalt für Umweltschutz) which gives me two or three relevant clues. Along with my intuition and a chat with a labmate, I conclude:

There is nothing systematic or conventionalised in the names of the isomers. They were likely named after the order in which they were isolated or their structures were determined.

Clues I used:


The $\unicode[Times]{x3B6}$, $\unicode[Times]{x3B7}$ and $\unicode[Times]{x3B8}$ isomers were synthesised last.
$\unicode[Times]{x3B1}$, $\unicode[Times]{x3B2}$ $\unicode[Times]{x3B3}$ and $\unicode[Times]{x3B4}$ were characterised first by a guy called Teunis van der Linden, after whom the active insecticide $\unicode[Times]{x3B3}$-hexachlorocyclohexane was named lindane.
Thereby concluding that $\unicode[Times]{x3B5}$ was discovered after $\unicode[Times]{x3B1}$ to $\unicode[Times]{x3B4}$.