This is tangential, but seeing the 8086 opcode table organized in octal groups made me realize the same applies for the instruction set of the Z80 (my home CPU). Your articles are always very insightful, thank you!
There's a whole lot of documentation on decoding instructions for the z80 which makes these octal relationships clear.
One good site is http://z80.info/, for example.
(I started coding on the Z80, and nowadays tinker with a single-board machine running CP/M on one.)
Compared to the 6809, the 6502 or the 68000, the biggest reminiscence of the Z80 is the difference between AX/CX/DX/BX having 8-bit parts and SI/DI/SP/BP not having them. Though the low/high parts of IX and IY were accessible on the Z80 via undocumented opcodes (maybe even SP? I don't remember).