Also seen a few commercial (and private) conversions where they fitted a single-stack mag like a Tikka or Rem 788 .308 mag inside the .303 case; OK for a 3 or 4 shot setup.

The one I have in "The Shed" came off a .243Win conversion of a No4, allegedly done by SAKO in the late 50s/ early 60s. However, I suspect that the closest it ever got to "SAKO" was possibly firing some of their ammo.