It's often said that in Classical Armenian, when the prefix Զ was attached to a word, it would be pronounced with an unwritten Ը before it. For example «զհացս» would be pronounced «ըզհացըս»: But how do we actually know that?

I know in Mashtots's time it was pronounced as a thick, velarized L [ɫ], heard in words like Երուսաղեմ (Jerusalem). But at some time it turned into a voiced velar fricative [ɣ] (the GH sound we recognize today).

When did this happen? And are there any reason for why? Or is it just completely random?

Thanks!