Jon;
My take is these overbore Lefevers were made that way for their gauge. In their early days I highly suspect that waterfowl guns accounted for a large portion of their sales. Early catalogs recommended using wads two sizes larger than standard for Black Powder use, but only one size for smokeless. My personal belief is that many waterfowlers shot brass cases & these two sizes larger wads was to accommodate the brass cases. Smokeless was more often loaded into paper cases thus the smaller wad. I think that a lot of the early guns were simply bored for use with the brass cases & oversized wads. Another thing I suspect but can't prove is that when they specified a wad they were speaking of its actual dia. Thus one over for 12 would be an 11ga or .751". In later years wads would be labeled for their intended use & a card or filler wad labled as 12 would actually measure to about an 11 & an 11 ga wad labled as for use in 12ga brass shells woud measure more like an 11/1 size. This would fit right in with Lefever's recommendations.
