In smaller cities (or countries!) it might be necessary to swap 260 and 240 used scarce parts. The 1979 260 and 240 instrument clusters look identical at front with small tacho in center and have an identical circuit board at back, but the 260 tacho is set for 3 pulses from the ignition coil negative terminal every crankshaft rev, but the 240 has 2 pulses per rev. If you have some fine soldering skills, the 260 tacho can be "re-calibrated" for 240 use by replacing its miniature 22K ohm potentiometer by a 50K ohm one. The tacho is easily removed from the cluster then from its plastic body to expose its mechanical internals and electronics. Don't be tempted to pull the needle on its face off to remove the face to get to the circuit board as its probably superglued and the mechanism is very delicate. The potentiometer is at the edge of the circuit board so can be accessed with the face still on but a small soldering iron is needed. Re-calibrate the "naked" tacho by temporarily connecting its + and - terminals to the car battery and centre terminal to the coil negative. Connect a reference tacho (automotive multimeter type)to coil negative and open throttle to 3000RPM and adjust potentiometer to get same RPM. Since the tacho is naked use some rubber (dishwashing) gloves to hold and adjust it since there is a post of someone getting an unpleasant surprize on touching coil negative. Re-assemble. If your 240 didn't have a tacho before there will be a black wire with insulated female spade connector hanging loose near the cluster wiring loom - its waiting for your tacho. Ensure that you have the right wire by doing a continuity check between that loose connector and coil negative terminal. Re-install cluster.

(For any electronics folks, early tacho chip (SAK215) & circuits are here)
http://pdf1.alldatasheet.com/datasheet-pdf/view/87653/ETC/SAK215.html
http://dzsc.com/pdf_down.asp?id=597072

sutherml at Adelaide, Australia
1979 244GL ay 518,000 km