The 5th wire is correct and was designed to supply direct power to the smoker to improve its performance. Be sure to get a 5th wire diagram as a traditional 4 wire is different. You can use the diagram from any late steam loco with 5th wire such as the 316 K5, 326 Hudson, 346 Switcher etc. ( I have emailed you a clear wiring diagram that covers all three reverse unit applications.)
The tab is a truck stop. Its uncommon to have it in that location, but that is its purpose.
Gilbert moved the reverse unit to the cab to allow easy disconnect of the tenders similar to Lionel. The cover should be plastic and was used to hide the reverse unit. Its debatable which is more unsightly, the reverse unit or the cover.
The trucks on that tender have been modified. Someone added the post that holds the slider shoes. The slider shoes were used on locos with the Air Chime whistle. They improved performance and reduced annoying static. If you look you can see where the post was poorly soldered into place instead of being crimped as the factory would have done. Later trucks all had the little pin hole in them for the stud.
Tin
