If Briggs used different piston ring groove dimensions with the chromed rings, it sounds as if they specifically didn't want people to fit them interchangeably with the plain cast iron rings. This may perhaps have been because they wanted to be fairly sure nobody would fit chromed rings, or steel rail oil rings, to an engine with an aluminium bore. I've certainly never been prepared to do so myself.
I remember the days when most car engine rings weren't chromed (yes, I'm pretty old) but the better grades of replacement rings were chromed. Most cars used to have their rings replaced once or twice before they were scrapped, in those days. Some models of engine tended to use more oil after re-ringing with chromed steel rail oil rings, compared with putting in a new set of original plain cast iron rings, but most models didn't. I haven't heard a convincing explanation of why this is so. Some of the old hands put it all down to break-in procedure when the new rings were fitted. (Back in those days, cars with plain iron rings always required a fairly complicated "running in" process to keep them from scuffing their new rings and performing poorly thereafter. Even now, it is possible to scuff modern rings, though it's more difficult than it was then.)
