What was written was probably decided at the local issuing office. I have the original tax discs for my 1925 Scott from 1925 through to 1960.
From 1925 to 1938 the discs were issued by Kent county council and from 1925 to 1932, they wrote MAX in the space provided for “Unladen Weight”. From 1933 to 1938, the wrote MAX in the area designated “Seat Capacity” with occasionally a line through “Seat” and cc added after MAX. That continued until 1950 with Essex and Middlesex issuing offices. A box labelled “HP” was struck through in all examples.
In 1951 to 1952, they wrote 596cc in the “Seat Capacity” box. In 1954/55 they had reverted to MAX again. I don’t know what happened between 1955 and 1959 (I don’t have the discs) but by 1960 the design of the disc had changed and they now had a box labelled “Cap or HP” and my disc had MAX written in that box. Another Scott tax disc for 1964 has 596cc in the box.