Flint and Pontiac
The 10th digit in the vehicle VIN number tells you which plant assembled your car. H= Flint P= Pontiac
All 1984 Regals were assembled at the Buick City Assembly plant. 1985 Model year production started in Flint but was transferred to the assembly plant in Pontiac when Buick City was taken off-line and re-tooled for the 1986 Buick Lesabre (H body).
So 1985's could be either Flint or Pontiac
1986 and 1987 Regal Production was done in Pontiac.
All Regal bodies 1984-87 were constructed at the Fisher Body plant in Flint and then delivered to either Flint or Pontiac (depending on year) for final assembly.
A side note: All Flint Built turbo cars got Hydraboost brakes, All Pontiac Built turbos got Powermaster brakes.
Dave