Does anyone know what 1 of 99 1967 GTX's that were built at Lynch Rd. in Detroit means?
Is it the first 99 built or a run of 99 at that plant?
Where do they fall in the order of manufacture?
Some, approximate, production numbers are know. When one says "1 of X built" it means that car is one of, approximately, 99 built for that year. As a exact production numbers are not known, it's a way to convey relative production numbers.
Production numbers by plant are not known, except for models produced at only one plant, so if a specific plant is mentioned and the car was built at multiple plants, then that person or source knows of at least that many cars from that plant.
For example, 69 Road Runner hard tops were produced at three plants; Lynch Road, St. Louis and LA. If I say the car is one of 1,000, it means the car is one of 1,000 similarly equipped cars spread out across the three plants. If I say it is one of 450 produced at the Lynch Road plant, it means I have data on at least 450 cars from that plant. As production numbers by plant are not known, it does not mean there were only 450 cars made at that plant.
For cars that saw a full model year run, the cars were spread out through the production year. Some package cars (Daytonas, Superbirds, AARs, etc) were only built during a certain time frame. So that 1,000 cars mentioned above could have been built over a 12 month period.