I happen to have a Senco sound tender set that is mint in the box and has all the paperwork and the elusive mixer box. I bought this from a friend in TTOS whom I did part time Ebay listing work for. He had acquired a collection of vintage 2 rail O scale. Most of it was from the 1940s and 50s. This collection had a bunch of Senco sound tenders, mostly 2 rail models but there were some 3 rail as well. There were 3-4 2 rail models that were mint in the box and a couple mint 3 rail models. There were also a bunch of 2 and 3 rail loose tenders as well. I’m guessing the collector must have bought a store’s remaining inventory back in the 40s.
This is the only time I’ve ever seen boxed examples of these. I have pdf scans of the paperwork for those who are interested.