Was this at all related to standard tubes using octal sockets? I can't think of any reason this would be personally, but I could be missing something.

On a PDP-11 there were 8 registers, R0-R7, with R6 being the (default) stack pointer and R7 being the program counter.

There were 8 address modes for each register.

So a src/dst instruction used up 12 bits for the src/dst+modes, a single register instruction used 6 bits.

You could load the registers directly at addresses 177770 through 177777 on the front panel. So if your program was 5 words long, you could load that into registers R0-R4, then load R7 with the address 177770 and execute.