I assume you're not familiar with the DBxxx series IPTV set-top boxes, with features in the later products like standard definition H.264 decode in hardware. There had multiple ARM cores which were used where possible, e.g. a core was used to independently run a specially modified internet stack. These are of course obsolete products today, since IPTV moves quickly, but bits of information can be found online if you dig around. The DB210 and 220 came first (note the dates in the URLs - this isn't the earliest possible reference on Pace's site, but it's the first I tried after mid-2004, when Pace doesn't list these products at all):
The IP215 is called an IP215 rather than DB215 because Pace kept changing their minds about naming things - you'll notice from their earlier products page that they call them "digital broadband media" devices, but later, explicitly refer to them as IPTV boxes and rename the whole RISC OS based range:
The earlier dated-based link shows the beginnings of the "IP" series naming. As a side-note, the IP4xx and (not listed) IP5xx series were somewhat ill-fated Linux-based designs using some rather unusual hardware. This was the last product I (or anyone else, I guess!) worked on at Pace Cambridge, with a few people kept on for a short while to support the products from a tiny office in Cambourne (IIRC).
Despite the similar names, the older MPEG 2 series differed quite a lot from the later H.264 boxes. Although the ARM architecture therein was compatible with 32-bit RISC OS, the rest of the chipset - for audio, video, infra-red and so-on - bore little resemblance to the likes of the ARM 7500FE-based DSL 4000.
ROOL brought along a couple of these STBs to Wakefield 2007, ROOL's first ever show:
Now, you will note that these products were appearing on the Pace site during 2005. I worked for Castle at the time and helped write some of the software inside them. I even went to IBC and demonstrated the products. Since Castle bought RISC OS from Pace in 2003 - according to press releases on both Castle's web site and Pace's web site - surely you can accept that the OS in these 2005 products was, indeed, "Castle" RISC OS?
There is surely no such thing as a "Pace branch" versus "Castle developed RISC OS 5". There is just Castle's RISC OS which was purchased from Pace. I don't see how there can possibly be any confusion or ambiguity here.