The X-300 has a horizontal-traverse shutter (see here) with flash sync at 1/60, meaning that it takes (a bit less than) 1/60 sec for one curtain to completely move across the image area.

I share Saaru's hypothesis that the shutter is stuck to some degree, in that the first curtain (opening the light path) moves a bit slower than the second curtain (the one closing the path).

Exposure time is controlled by the delay between the first and the second curtains starting their motions. Assuming that they move with the same speed, there's a constant time that every spot on the film gets exposed. And that seems to fail in your camera. The second curtain seems to reach or even overtake the first, leaving no gap for light to reach the film.

This should be less of a problem with long exposure times.

For a manual "inspection", it might be possible to "shoot" some exposures (1/250, 1/500, 1/1000) without film and with the back cover opened. Your eye can't follow the curtain movement, but you'll be able to see whether all the film area gets some light.