I was too impatient to wait for my M42-Fuji X mount adapter from Aliexpress to arrive, so I printed one for temporary use! It's got the locking feature from the X mount,
A Vintage Alternative With Fuji X-E1 by Meng Yeap
K&F Concept M42 to FX Lens Mount Adapter M42 Lens to Fujifilm X-Series Mirrorless Camera Adapter - KENTFAITH
Amazon.com : K&F Concept M42 to Fuji X Lens Mount Adapter for M42 Screw Mount Lens to Fujifilm Fuji X-Series X FX Mount Mirrorless Cameras with Matting Varnish Design for Fuji XT2
Techinal Universal Adjustable Aluminum M42-FX M42 M 42 Lens for Fujifilm X Mount Fuji X-Pro1 X-M1 X-E1 X-E2 Adapter Ring Camera Accessories Kit - Walmart.com
K&F Concept M42 Fuji X PRO Lens Mount Adapter for M42 Lenses Threaded for FX Mount Mirrorless Camera with Matte Varnish Design|Lens Adapter| - AliExpress