Mike
I took a really close look at mine, and there is no "triangular" area you mention. All the way around the frame and body, it looks like a single casting.
This is the tender that came with a Spectrum 2-8-0 I purchased a while back...
Mark in Oregon
Sorry, I thought you meant the Spectrum tenders that were sold separately. As others have said, the tenders that came with the early locomotive releases were a bit different.