PROVA means 'test-driving' on an Italian plate, they were the Italian equivalent of the red "trade plates" used in Britain, enabling unregistered cars to be driven on the public roads.

MO is for Modena; the home district of the Ferrari works.

Cars were given those plates (or painted on the body) so the Italian police would not pay attention to what speed they were travelling on public roads. When the cars were entered in races, the "prova" plates were often still on them.

They would take the cars for a quick shakedown up and down the nearest Autostrada from the Ferrari factory in Marenello

They were still painting the PROVA markings onto the Formula One cars up until at least 1967.