This is the ’22 Mustang GT California Special Edition undergoing testing in Dearborn, Michigan, with masked logos and badges on the grille, door and decklid.
Based on the engine note, the convertible musclecar is still powered by Ford's 5.0L Coyote V8 and is mated to a six-speed manual. The only visual difference that we can find is a different set of wheels that is similar to (but not the same as) the units used on the Mustang Bullitt that was discontinued after the ’20 model year.
Exactly why Ford is updating the S550 Mustang so late in its lifecycle is puzzling, but it may just be a play to wring out some additional bucks before it goes to pasture.
Photo credit: Brian Williams, SpiedBilde