This is a housing to build a tachometer/speedometer for a Rong-Fu milling machine (also sold by other brands).

You will need:

• both parts from this thing
• tachometer from banggood or other source: https://www.banggood.com/Red-LED-Tachometer-RPM-Speed-Meter-with-Proximity-Switch-Sensor-NPN-p-928692.html?rmmds=myorder&cur_warehouse=CN
• power supply (I power mine from my VFD which can provide 24V/100mA)
• 2 bolts M5 and washers
• 4 countersunk bolts M3
• a small and strong magnet