The -k' plural suffix in Armenian is well known. What is less known that in old Armenian it's accusative case was transforming into -s. Because toponyms are used frequently in this case so sometimes a toponym with final -k' was transforming into -s. And was used in this form.
Masis Mountain ( so the root is the Masi )
Kars ( the city Kars)
Moks from Mokk' ( South of lake Van)
Acharyan also suggested that Tp'lis (modern Tbilisi) was possibly another such a example.
Moks is remarkable because it is attested in Graeco-Roman sources in the form of Moxoene. Which implies that this shift typical to Armenian language is attested in those old texts.