I do now that when I lay my 44.5 next to one of my tip ups the plate is identical, so it would seem stevens used at least some plates across models. Sure wish I could find one of those schuetzen plates for the tip up.

You know I've never taken plates off my Ballards and my Stevens to compare, but they do look awfully close. Of course all the makers did stock final shaping with the buttplates in place, so none of them will be a perfect fit, even within the same maker. 
I'll pull one off of a Ballard and a Stevens and get back with what I find. My little 3f does look similar to the plate my engraved Stevens.
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Well it seems the theory that they were all built by one source is not holding water. After looking at both the Ballard and Stevens buttplates there are a couple major differences.
The Stevens has a daintier upper prong, while the Ballard in the equal size plate has a more robust prong. Also, the Ballard's return on top is more pointed, while the Stevens is a gentle rounded top return. 
Smaller things are the general curve, which is close, but not excatly the same. So it seems they each had a general similar look, but different enough to not be from the same mold.