There is also the difference in the bolt on toe rings that exist. the Model 5 series 1 toe ring as below has 8 locating tabs (that stick up) around it.
And the later one for the utilities, it has 6 locating tabs around it (both have 2 front larger tabs that clip over the base), as below.
Blue I'm not doubting myself, I am doubting the acuracy of the sketches in the Victa manuals, just like the Model 5 ser 3 with axel extensions on the base, this is just incorrect. I also understand what you mean about the indentations in the base of the Model 3 for the clip on base and some pics would be great.