[masswerk]

The idea was more about the arrangement of the character set, which isn't always obvious on the VIC-20 or the C64, because of the differences in keyboard layouts as compared to the original PET 2001. (E.g., both the VIC-20 and the C64 are lacking the numeric keypad and what are adjacent blocks on the PET is dispersed over a linear arrangement. So there's no discernible logic in the code, nor in the keyboard. The same is true for several other keys, like those around the space key of the PET.)

Regarding the number of actually assigned codes, mind that the basic code, regarding unique displayed characters, is single case (6 1/2-bit) in two banks (MSB set for shifted characters => 0x60 x 2 = 192). However, the mirrored regions are actually unique codes in BASIC (as far as CHR$() and ASC() are concerned).

Sorry, if I failed in making this clear.

Edit: Arguably, the ortho layout of the PET 2001's chiclet keyboard also helps in identifying logical blocks of graphics characters.