Well, some modifications and upgrades were major, others not so much.
Generally speaking, an “initial” Tiger I is one with the original design. This would include the taller cupola, smoke dischargers, the storage bins on the side of the turret (some were on both sides), two headlights on the front of the hull, Fiefel air cleaners on the engine deck, and wide, rubber-tire-havin’ roadwheels. No Zimmerit (hadn’t been invented yet).
Early-production Tiger Is have all that, but they’d settled on a turret stowage bin that fit properly on the back of the turret. Things like shovels and other tools were beginning to be moved around to better locations on the hull deck, and some vehicles had only one headlight instead of two. AchtungPanzer can tell you where to fix a shovel or a track tool by specific date. This is the version seen at Kursk, where the smoke dischargers were beginning to be removed in the field due to safety issues. These tanks were also Zimmerit-free.
As Tigers were continually produced, changes continued to be implemented. Sometimes you see one with the early-style cupola and steel-rimmed wheels, and other times you see the late-style cupola and rubber-rimmed wheels. These tanks are generally referred to as “mid-production” Tigers. They may or may not have Zimmerit, depending on their date of manufacture.
A late-production Tiger I is one with the lower cupola and all-steel roadwheels. It will have no smoke dischargers (with a few exceptions), no Feifel air cleaners, one headlight (if any), and will be covered with Zimmerit.
Final-production Tiger Is only differ from late-production ones in minor details, like the grooves cut into the angles on the cupola - stuff like that. The end of production for Tiger Is came before the end of Zimmerit use, so all late- and final-production examples were zimmed up.
Hope that helps. Also, I hope it’s correct - LOL