This is a standard detail in eighteenth and nineteenth century softwood flooring, here in the Uk.
Ged Gardiner wrote a fascinating article for the Georgian Society, in which he dismissed the currently accepted theory; the rebate was made with a sash fillester plane, ( as described by Walter Rose, in the Village Carpenter). Instead he postulated that the rebate was made using a standing fillister, or a rebate plane with a depth gauge, ( the depth being gauged off the visible face of the floorboard). In practice, the carpenter would lay the floorboards face down on a flat plank, then plane from above until the depth stop made contact with the flat plank, and the rebate on the floorboard would now be cut to a standard depth. Then using a hachett or an adze the waste could be removed to this depth, once the position of the joists were known.
Hope you can follow this.
