The ECU serial code is printed on the underside of the ECU. It begins with SH i think and then 7 or 8 digits. Be careful as the ECUs are different for the 91-93 clios as they are for the 93-96 models. Find out the ECU number from his 19 and if its the same as your clio it should be fine. I dont know if the ECUs are the same for the clios and 19s, but if your car year is different from his youve no chance anyay. The later clios had fuel cut of switches, the earlier ones had cold start injectors, and the ECU needs to be different for each.
By the way mine is a 92, but with a 93-96 ecu so dont assume that if the registration years are the same the ECU will be the same. The ECU on your clio should be underneath the vehicle jack, which is located at the back of the engine bay to the left underneath the front windscreen glass. Its dead easy to get it out and have a look at the serial code.