This is a precision test cassette, recorded full track (the full width of the tape) on a special recording setup using high quality NOS Maxell Communicator C60 blanks. 

Each cassette has a unique serial number (so one you receive might have a different number compared to one on the photo) and is fully tested with the result recorded on a computer. The residual W&F is below 0,02% WRMS, 0,04% DIN wtd.

This cassette can be used to check and setup the tape speed, and also to check W&F for the full length of the cassette (30 min for C60)