I just was looking at the I/O device code assignments in the 1973 DECsystem-10 System Reference Manual, and happened to notice the entry for the Type 270 disk file used on the PDP-6. PDP-6 and PDP-10 device codes are three octal digits, of which the third digit can only be 0 or 4. The device code for the Type 270 is octal 270. Coincidence? :-)