Frame stamping

I was searching for images of the 6v Honda ST70 on the web the other day when I suddenly realised that some of the dax's I was looking at had a slightly different frame moulding/stamping on the gear lever side by the "shield" decal. Instead off a "V" shape below the decal it just has a straight line (see below image). The other side of the bike still has the normal "V" shape stamping. At first I thought it was just on the K3 models with the flame decals but some have the V shape and some don't.
Can anybody shed any light as to why and when Honda altered the frame this way.

Hi
My 1969 ST50 has the diamond pressing on both sides ???????????
Tufty