The 2SD1908 transistor is documented in this 85.8 KB PDF, which the index files as a schematic, although a JIS part number like this normally indicates a datasheet. The 2SD series groups NPN transistors for low-frequency work such as audio amplification, regulation and switching. Technicians open it in any PDF reader to check pin order, voltage and current limits and dissipation before selecting a substitute. The name states no other variant or revision. Compare the printed marking, often D1908, and the package with the drawing.
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.