Works with a plain adapter
Leica Screw Mount (M39) sits 11.10mm further from the sensor than Fujifilm X. You need a 11.10mm adapter, but it can be a plain one — a hollow metal spacer with no glass in it. The lens still reaches infinity focus and nothing is added to the optical path.
Coverage & framing. The lens covers more than the sensor needs, so the body crops into the frame — roughly 1.5×. A 50mm frames like a 75mm would on Full frame.