My data plate engine code is a U 162 hp but my engine is a M 182 hp
4bbl duel exhaust. Could this be a dealer upgrade or did it come from the assembly line?
How can one tell if it is the original engine it is a 55 Fairlane Town Sedan.

You can't actually tell if your engine is original because Ford didn't tie the engine number to the VIN back then. About the best you can do is compare the vehicle build date from the data plate to the engine block casting date. If they are within a month or so of each other your engine is likely original.

since all fairlanes had duel exhaust it could be as simple as upgrading to a 4 bbl.check the head casting numbers that should tell you something.

I junked out a ‘U’ code 1955 Country Squire that was loaded with the 182HP Special, automatic transmission, factory A/C, power brakes, power windows, and power seat. From all indications, it was the original engine but did have the ‘U’ code on the VIN instead of the ‘M’.  The block and heads (ECL-B) were all date correct for the car so there’s a reasonable chance that this car did leave the factory with an incorrect engine code.

Lorena, Texas (South of Waco)