The BMW M1 and the Porsche 935 I found that the PPR 1500, and the 1420, 1423, 1425 all fit the car,
First the 1400 series, these are the tires for the slot it F-1 rims, the 1420 being the smallest in diameter and the 1425 the largest. These tires fit the rim rib perfect but fall short in the width, they leave the chrome from the rim showing on the inside and outside of the wheel, however the tire slips right on with no fuss.
The 1500 tire is the perfect width but requires a wegie to keep the tire from side walking, it also fills the wheel well as the stock tires do. you also need to true the rim rib down a bit to get the crown out of the tire, this is in most cases a good thing, the rim will have to be trued, so with having to reduce the overall diameter a bit will give you more room to get even the worst rims true.
For these cars the tire allowance will be the tires listed above, I think that some may use the 1400 series for the quick fit, and some will use the 1500 becuase they have more room for an exact tire true, and to use the full width of the rim.
The Lancia Beta and the Ford Capri use the PPR1010 and are a direct fit (these are a listed tire for the car)