Funny you should post the question, I was asking myself that exact same thing, but by looking closer at the internals, it seems ZSLA1 is some kind of single tube gain stage.
Something like the a Zkit4 + Zkit 5 except with a bigger transformer (overkill) and a pot for volume control.
But that's only an educated guess i'm making, please don't take it as fact.
Regards
Nick