Differences between the PDP-11/15 and PDP-11/20

A few months back you might recall the road trip I did to recover a Foxboro FOX-2, a rebadged PDP-11 that ran the furnace oxygenation plant at the BHP steel mill at Port Kembla here in Oz during the 1970s. I had thought the machine was an 11/20 with a custom FOX front panel that (apart from the Foxboro lime green colour trim) had only 16 address switches instead of the 18 found on that machine, and that there might be two more vacant switch locations on the front panel PCB to be found lurking underneath when it came time to disassembling. But having just seen the PDP-11/15 up on eBay, I now believe that is what the FOX-2 really is. This page (http://gunkies.org/wiki/PDP-11/20) reckons the /15 is just an OEM version of the /20 but I'm wondering if there are any more differences apart from the lack of the two high address switches? (Presumably the /15 could not run one of those KT-11 paging units as was seen on that recent eBay 11/20) Steve.