The Ferrari 312 P was a Group 6 Prototype-Sports Car used for racing in 1969 and 1970. The new 1971 version of the sports prototype came with a flat-12 engine, often referred to as a boxer engine. Many publications added the letter B after the P of its name to indicate its engine type, but this variation was never officially sanctioned by Ferrari which simply called it the 1971 312P. (Wikipedia)
Definitely not a custom...