For what its worth.
Before the adoption of SG, the majority of railroads in the Official Guides (circa 1870-1885) was 4'8" at a minimum up to a maximum of 4'10.5" with the majority being between 4' 8.5" and 4' 9.25".
The notes at the end of the timetables said they interchanged with each other and if tire widths are wide enough, there should not be much of a problem. However, interchange used then may not mean what we mean now. It might mean they could transfer goods directly from one car of one gauge to another car of a different gauge. In other words, there is no need to hire a wagon transfer company to move the load from one railroad to another.
Doug vV