Amazon.com : (2 Packs) M39 to M42 Adapter Ring, 39mm to 42mm Lens Adapter, for 42mm Focusing Helicoid : Camera Lens Caps : Electronics
Still waiting on the helicoid, but got the M42 adapter for my square aperture Schneider Kreuznach Enlarger Lens. A few backyard test shots : r/VintageLenses