Mr. Trzoniec describes the .225 as "semi-rimmed", and it does appear to have a shallow indentation forward of the rim. I'm curious as to why the indentation is necessary, unless it's to offset the smaller rim to insure reliable extraction. Interesting. Seems to me this cartridge could thus be used with either a bolt or lever action. Makes me curious as to how this design was arrived at, and what ideas were being kicked around at the time...
The usual reason given for calling the .225 "semi-rimmed" is that, even though it has a clear and obvious rim, unlike the Swift, it still headspaces on the shoulder of the Model 70.
That old article I referenced made the point that it could be treated as rimmed for use in actions made for it and difficult to set up with rimless extractor.
Why Winchester went with such an odd design is probably buried in the minutes of design meetings of long ago.
Maybe their success with .243 over .244 went to their head and they thought they could do it again.
Turns out they would have done better to go back to an earlier era when they followed the .257 Remington Roberts with the .257 Winchester Roberts; and just put their headstamp on .22-250 to compete head to head with Remington.
Ed Matunas once wrote that although they were still selling .220 Swift ammo, they would never make another Swift rifle, they were too scared of the stick-slip phenomenon in eroded throats causing kaBooms and getting them sued.
The little groove ahead of the rim is common in rimmed cartridges.
It is to give a clean flat front face to the rim, without a radius as would be left by the drawing die. Easier to just cut a groove than to try to come out exactly flush with the case body.