Tolkien stated that the combination [ŋx] (which he spelled “9H”) became [xx] (spelled “HH”), which he described as “phonetically XX, [the] long back voiceless spirant” (SD/420). This combination could only arise via nasal-assimilation from [nx], since nasal-infixion only occurred before stops and [z].

Since Tolkien wrote the result as “HH” (as spelled in primitive Adunanic) instead of “KKH” (as spelled in Classical Adûnaic), this was likely an early change. This change could have happened either before or after spirant [x] became breath [h] except before [s] and for double [xx], since the combination [xx] remained spirantal, but given his use of the primitive-Adûnaic spelling “HH”, was probably before.