Although Yamanaka wasn't the proven fighter he is today you can see a lot of the same traits we see in today's version of the Japanese fighter. That includes the same under-used jab and the repeated, and relatively blatant, attempt to measure his foe for his straight left hand. It seems even back then Yamanaka had full belief in the power of his left hand and was happy to find a way to land it to head or body.
One thing to note about this fight is the contradicting records of the Wanpadechsuk. Boxrec.com have him in as a 0-3 fighter whilst Japanese TV have him listed as being 7-2 (3). We tend to believe the Japanese on these sorts of things due to known inaccuracies in Thai records in the past.