Straco and the Mystery Train 2

This is a followup post to my 9/6/16 post, Straco and the Mystery Train. Some odds and ends had come up for auction on eBay, and just by comparing the photos, I guessed that they were made by the same Japanese toy company that made the Straco Express. Those particular items were priced too high for me to bid on -- but then another example became available that was within my price range (about $8.99). 

So I can at least make physical comparisons between the box cars. I think the evidence is clear: these box cars were made by the same company. 

The Straco Express box car (top) and the mystery set
box car (bottom). Identical construction and chassis lithography.

Straco Express (left), mystery train (right).
The ends were something of a surprise. There's no real reason
for a graphic redesign. All that was necessary was to change
 the red color plate to blue. Note that the couplers
are also identical. 

And here they are together. A colorful (and patriotic) pair of boxcars!

Some questions remain, of course. Who made these sets? What pieces were in the State of Maine boxcar set? Who was this mystery set made for -- was it also Fred Strauss and Co.? 

Based on this, though, I'm confident the caboose I saw in that auction is also a companion piece to the Straco Express caboose. That locomotive, though...

What's missing from this set?