A family pop group need transportation to Caesar's Palace, Las Vegas. They purchase a used 1957 Chevrolet roundback school bus from Al's Used Cars (actually bought by the show from Orange County School District). The family paints the bus in the distinctive geometric style of the Dutch painter, Piet Mondrian.
The rear of the bus displays both the license plate NLX590 and the sign 'Careful, Nervous Mother Driving.':
Evidently a number of ostensibly identical buses were used - this one's slightly different:
When the show ended, the bus passed through many hands until it was abandoned in a parking lot at Lucy's Tacos, East Los Angeles. It was finally scrapped in 1987. David Cassidy (Keith Partridge) toured in 1993 in a look-alike bus. Another copy of the original bus can be seen at Universal Studios in Florida.
Christine Petix reports a 1999 replica of the original Partridge Family bus in Flint, Michigan. I would guess this would have been used in the 1999 movie 'Come On, Get Happy', which told the story of the group.