Further to the discussion just looking at my parts book which is for an A7 KX 125 not A6 as I earlier stated but printed Aug. 22 1980, the accessories page lists
Part# 58029-1008-21 PLATE-NUMBER, F. BLACK (ROUND PLATE) SPEC CODE AL, UK
AL= All Market, UK= United Kingdom
Part# 58029A-1009-6W PLATE-NUMBER, L. GREEN (SQUARE PLATE) SPEC CODE CN, US
CN= Canada, US= United States
So therefore the square plate is US and Canada only but a round plate can also be used. I guess the quality control level in those days varied from day to day