Mark, if I recall you posed the very same question months (years?) ago on this thread and referenced the very same green Ebbets Field/"Da Bums" pennant. Observations like this one inspired me to start looking at these companies further in search of some answers. That effort eventually led to my BLOG.
So, thank you. (I think I even used your photo of that pennant in my Trench post.)
Anyway, I'm working on an ADFLAG post that I think will answer all of our questions. With that said, I hinted at what I think the answer is in my last post:
https://pennantfever.weebly.com/blog.../trench-mfg-co
There are plenty of examples of instances wherein it appears Trench's designs were copied by their competitors. The chief offender was ADFLAG, but also ASCO and to a lesser extent, Keezer, were all guilty. I don't suspect there was any type of partnership between these companies. The artwork was simply ripped off--plain and simple--because it was in high demand and it wasn't legally protected like it would be today.
There's a Cincinnati Reds stadium pennant comparison in my last post you may want to re-visit. It looks a lot like the aforementioned Ebbets Field hybrid you reference. The artwork of Crosley Field is identical between the two pennants ... but for one small difference: Only Trench placed their mark beside the image. That's significant. Since ADFLAG was then secreting their name within their artwork, we should ask why they omitted it on this particular design? The most logical answer is that this wasn't their artwork, and placing their name in it would have crossed the line.
Like signing your name on someone else's painting.
In sum, it looks like ADFLAG reproduced some of Trench's original artwork on their pennants. To make the designs their own, they then doctored the pennants up with their own text, using their own font. The end result are these Frankenstein-like pennants that represent an amalgamation of two companies' work.
And there have to be other Frankensteins out there! I suspect they will involve stadium pennants made in the mid-1950s. By the 1960s, it looks like ADFLAG began making their own stadium renderings; however, when compared together, they're still
very similar to Trench's designs, like the two Chicago Cubs/Wrigley Field stadium pennants shown in my last post.
Much more to come.