P/N 89661-50303 superceeds P/N's 8966150300; 8966150302; 8966150304
Your ECU may have been replaced. It depends on production date of your chassis and the date that the supercession of the part occurred. IOW, your chassis may have been in-process on the production line with the 50302 when the 50303 was put into production. Odds are that it was replaced at some point after production. I got the old service records from the local Lexus dealer for my car when I bought it used from Carmax in 2008. These records may some of your questions.
Regardless, the ECU you want is the latest part number. It will have the most recent calibrations.