plc 5 internals

hi all
I know plc 5 inherited octal system from plc 2. But why was octal addressing used in the first place? why not decimal or hexadecimal? plc-5 internally only understands binary and other codes are for us humans…
Does anybody know internals of plc 5- is it 8 bit on 8 bit bus but addressing 16 bit memory space - is it right? does anybody know of doc showing internals of this impressive plc?what makes it so robust? i know s7 only and learning allen bradleys using plc 5 - i know its old but then i will move to slc and logix Any comments welcome
My understanding, which is incomplete and doesn't extend to the actual structure inside the processors, is that the octal addressing is because it shares some hardware (chassis, IO modules) with the older PLC2, which was an 8-bit processor. The PLC5 data tables (Bx, Nx, etc.) are not addressed in octal; just the I/O. That's part of the PLC2 legacy hardware.