When we discuss "matching-number" cars, we're talking about originality. Do the engine, transmission, and rear-axle differential casting numbers and date codes jibe with the assembly date of the vehicle? Normally, these numbers should fall 30-60 days prior to the assembly date of the vehicle. Not all classic Mustangs have the original engine, for example. Even rarer is an original transmission because so many were replaced with new or rebuilt replacements. But it's nice to know where you stand before a purchase-especially if the seller is claiming a "matching-number" car.