I fired a query to my local connections...
when they changed to the Howa, as with everything it was a gradual process (like when the British Army switched to MTP, first people working in the supply chain, last the combat troops...
). Technically after the type 64 come into service the army also changed uniform from US style to the Type 65 Uniform (basically US Vietnam style, but dark olive). The webbing followed pattern.
so you have 60' style US uniform, first uncovered, then covered M1 Helmet, Howa 64.