A bit of history, skip to the wiring below if needed.
The Year on both my engine and tender shells are marked 1997 (As old as I am), probably the latest & last tooling type before Bowser stopped production of their steam loco kits around 2010. It should be at least a 34-year upgraded tooling difference from when they first bought the assets from Penn Line in 1963, who produced their own T-1 (Variants had either Spoked or Boxpok Drivers, both inaccurate) before bankruptcy.
Earlier Bowser variants from Penn Line’s tooling always came with a brass sheet low-side tender and double smokestack engine shell, some engine shell variants had cast-on air pumps, separate screwed-on ladders, or full casting where you needed to drill the holes for the details yourself.
Completely different from the newer single smokestack and die-cast high-side tender, and factory drilled holes.
Some old eBay listings on Worthpoint that show the design of the instruction cover for the earlier kits on the third image: