That is a bit paranoid. Considering the huge investment needed for an NOC, I doubt they would risk their reputation swapping a few components. They would likely have standardized the ones they use.
However, you can be sure you get your own components back. Most all of them have their own serial numbers, in addition to model numbers, and many have the date of manufacture silk screened on the part. Many model numbers indicate the date, or range of dates, on which an item was manufactured. Additionally, at the rate computer components change, if a component was swapped, it would be unlikely to have the same model number.
If you record the model numbers, serial numbers, and date of manufacture of all your components before you send off your server, you can check the parts when you get it back. be sure to note any replacements you might have to make if a part fails.
There are also server chassis with chassis intrusion alarms that alert you if the case has been opened, but recording the numbers would be more effective.
Still, it is highly unlikely you would get anything other than your own server back, with all its parts intact.