I'm still unsure whether Miller sold the SE-100 as a complete set to make all three engines, or as a single configuration set-up, and then sold the parts to make the other two configurations separately?
Possibly they sold them both ways, but to my mind, the relatively small bundle of parts needed to make the other two engines couldn't have been very expensive or it wouldn't sell, so by the same token couldn't have been a significant profit maker as an after-sale add on. It does make more sense to me from this distance, to charge a bit more up front for the total package of the three-engine steam plant, with all parts included. Even if done this way, there would likely have been a packet of parts inside the box, and a tag like that one shown, would likely be there with those parts!
That being said, it does amaze me that so many of these engines are found in just one configuration, and not always the Beam Engine either, without any of the other parts to produce the other engine configurations. I actually have one that I received as just the horizontal mill engine, but it did have all three sets of plans with it. So, perhaps it was available in any one of the three configurations, and then you could buy the parts for either or both of the other two later. Also possible, that they might include the blueprints for all three configurations just to tempt the purchaser into buying the other needed parts.
Most likely of all, or so I think, is just that owners of these engines tended to build them into a single configuration and leave them that way, eventually losing/misplacing the other parts?!?!
Yet another steam mystery lost from the ages!!!
Meanwhile, here's my most recent acquisition along these lines, with an interesting Red repainted base:
https://www.officeofsteamforum.com/general-discussion/and-then-there-were-four!/