The pictures are not clear. I cannot make sure if it is an electronic tachometer or a mechanical one. Please take clear pictures of the back of meter and post here.
The only way it can be fitted to a CD70 is that its an electronic tachometer and does not require a dedicated connection. Only in that case, one of the wires connects to another wire that has a clamp at the other end which is put around the spark plug and the other to ground. Personally I have never seen such a tachometer.
A tachometer(RPM meter) is a device to measure the number of rotations per minute (RPM) and has several types. Even fan manufacturers have a tachometer that has a rubber cup at one end which is pressed against the center of the fan and it gives the RPM of the running fan.
For setting the idle speed in motorbikes which do not have tachometers, the mechanics use an electronic tachometer which connects to the spark plug lead and it gives the RPM based on the number of electricity pulses going through the wire. A similar, more common device is used to set the timing of the spark, called a timing gun, which gives a flash of light when the current passes through the spark plug lead.
All bikes manufactured in Pakistan which do have tachometers, have the mechanical type which uses a cable similar to the speedometer cable, no electric/electronic connections except for the wires to the light bulb.