The 2sk2578 is the part described in this 34.5 KB PDF, filed as a schematic. JIS part naming uses 2SK for N-channel FETs; the name gives no package, maker or application details. As a short datasheet it should list pin functions and maximum ratings, which technicians rely on when checking a transistor for a gate or drain short and sizing an equivalent. Open it in any PDF reader, and confirm the marking and footprint of the installed part agree with the document.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @cyberlanelectonik. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.