Ok then it definitely defines the date then of a unit. eons ago when I first had started compute exchange in Phx we had one that just said pdp -11 as I remember it came from Sandia Labs. Currently we have a 11/20 Ed# smecc.org In a message dated 2/22/2016 1:12:16 A.M. US Mountain Standard Time, pontus at Update.UU.SE writes: The PDP-11/20 was the first PDP-11. I believe the number was added when the other models came out, to discern them. /P On Mon, Feb 22, 2016 at 03:03:17AM -0500, COURYHOUSE at aol.com wrote: differences??