As usual Maxim is on spot. The 21j movt rep are big tell, they scream FAKE all over the place, in a real chrono the running secs is one of the subdial and the center secs hand is for chrono. In the 21j the running secs is the center hand, so it's running all the time. Also the Day-Month-Military time subdials are big let down, in a chrono the min & hours counter are lineup in a vertical position, in the 21j the subdials hands are going to be pointing everywhere.
As Maxim mentioned, if you want to save some money, look for a Quartz version, they are not that far from the gens and are much more cheap than the A7750 chrono automatic versions.
Let the 21j movt for watch without chrono function, those just with H-M-S. The 21j is workhorse, so it's great on those models and you will save money.
If you want to learn a little more about movts, go to the Movement Q&A section (HERE)