I have a Model 119 sidelock in 16 gauge and it is on a 16 gauge frame. I have two 20 gauge boxlocks and for sure they are on 20 gauge frames. I do have an older export 12 gauge sidelock with extractors and it is definitely a larger frame than the 16 gauge.
On guns built to a price point for a particular importer, your mileage may vary but one of my 20's and the 16 were made for me and imported through Aspen Outfitting, and they are on scaled frames appropriate to their gauge.
Maybe Wieland's book has something in writing on this.
Bruce
