I have my undertray off at the moment ready for a weekend oil change. The only turnbuckle screws that come out completely are the front lip four. The ones on rest of the tray are captive.
If you're having to drill or do anything with the tray other than remove and refit then it's either misaligned or the wrong tray for the car.
I can't see even the later cars not being fitted out with a tray or at least the fixing locations. Why would you make such a minor change nearly at the end of a production run.
Always locate the front lip first. The other six then just fall into place. Copperease grease is good and makes for easier removal next time