A standard kit lens from the Fujica ST series of SLR cameras. The ST series of cameras used the M42 lens mount. This lens seems really well made, all metal.
X-Fujinon lenses use the Fuji Photo Film Co. X Bayonet mount, not to be confused with the new Fujifilm XM Mount for their mirrorless digital cameras.
This lens, the 50mm f1.9, had to have been the standard kit lens for the Fuji line of lightweight (plastic) SLR cameras. There is one cheaper, the 55mm f2.2, which can be found with both the earlier M42 mount, as well as the X Bayonet mount.
Overall, the mechanical quality of the lens seem very low. Except for the lens mount itself, and the aperture, everything else appears to be made with plastic, including the focusing coils. Focus is very loose on this particular lens. The glass seems to good, and overall contrast is good.