There never was a 1.4x teleconverter for M42. Plenty of 2x, but they were not the best quality. I do use a lot of M42 lenses with my Fuji X series mirrorless camera. One way to use M42 lens and have a 1.4x teleconverter is to use the Fujifilm XF 1.4x TC WR. Mount it on camera, then use a M42 to Fuji X adapter, then mount the lens on the adapter. It is designed for a handful of Fuji XF zooms. It'll all be manual focus, of course, but it works.

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