This seems a bit dubious to me. I don't think a variable symbol width equates to mixed radix. Nobody calls UTF-8 mixed radix even though some symbols require 8

This seems a bit dubious to me. I don't think a variable symbol width equates to mixed radix. Nobody calls UTF-8 mixed radix even though some symbols require 8 bits and others 16. You aren't treating these values as algebraic numbers. While you can perform some types of arithmetic on (x, y) vectors in a mixed-radix form (addition, scaling, etc) and same for HH:MM:SS, it makes no sense to add the binary/mixed radix representation of two MIPS instructions. But maybe the definition isn't this picky. They certainly feel like different concepts to me though.