A versatile system for building a versatile bike.
Available as complete dropout sets and/or separate parts, you have options without buying parts you don't need. Thru-axle now, horizontal or vertical later.
The steel receivers (20mm where the stays attach) are the foundation of this system. They are what you weld or braze into the frame.
Pro-tip: Since thru-axle dummy axles are not cheap, one work-around is to buy a complete set of thru-axle dropouts/receiver and a pair of vertical dropouts. Build the bike using a standard dummy axle, which most builders have, and then swap out the vertical dropouts for the thru-axle dropouts.