Hi John, all
"John Turner" wrote
plug/socket, although Hornby in their wisdom have
NMRA specifies 4 (large scale), 6 (small), 8 (medium) and 9 pin, and 21 pin is kind of under discussion, mainly because there are also alternatives proposed, like the 8/16/22 one ...
see also (bilingual page, English right hand side)
as for the 9pin, Athearn equips their (US) models which are labeled Dcc-Quick-plug with 9-pin (yes, 9 pin) sockets, matching the NMRA 9pin specs. (but not the 9pin NEM specs ... oh dear ...)
cheers/jw (jens)