ALRO circular Decibel Slide rule full set small PTT 1970 Holland

This small round slider rule is marked PTT a Company from Holland (The Nederlands) and is two sided. It is in very good condition with perfect chromium plating on the centre of both sides. But there is tiny piece missing from the plastic outer ring just besides the cursor). It comes in the original plastic sleeve that is also in great condition with paper instructions (in Dutch). I assume it is from somewhere in the 1970’s like it’s the bigger model also for sale here. It is also made by ALRO from the Netherlands / Holland. Both sides have a see-through curser with hairline. It measures ca 8 cm or 3 1/4 inches in diameter. Check the attached video on how to work your vintage man tool : slide rule operation instructions!