Schematic in PDF form for a stereo preamplifier with a loudness control, a common DIY audio circuit that boosts bass and treble at low listening volume. The RAT NET tag in the name is probably a source or author label, not a model number. Technicians and hobbyists use a sheet like this to trace the signal path, check the tone and loudness network values and find faults such as noisy channels. Compare the supply voltage and transistor or op-amp types against your own board before swapping parts, because DIY variants differ widely.
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 @pdf_servicemanual. 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.