Short answer is: I used the same date code on my barrel so the two would match. I did this under the assumption that the majority of what is out there was put together at the same time in the same time frame.
Long answer is: There may be someone here that has more knowledge behind the application of date codes and how they would be applied. I dont know about 416/417s but I dont think it is uncommon to have HKs with different date coded furniture (IE 91s & 93s buttstocks & handguards, MP5s with buttstocks and forearms) it is however desirable to have consecutive parts and pieces but in a world of piecing things together you may have to shop around. I do think barrels and receivers if they are barreled at an HK facility would be date coded the same. To my knowledge on 416/417 none of the grips, stocks etc are date coded. They are categorized by version and most can be differentiated by changes in characteristics.
Maybe @Marine0303 can offer insight....Sir, any guidance??