I don't know about legally speaking, but in terms of my on tiny set of "poster ethics", I think that if this border was taken from an Art Chantry poster (not all that familiar w/ your catalogue) then that sucks. Stealing and "reappropriating" artwork/photography can be okay with me if done well (an arbitrary statement, I know), but I don't like the idea of stealing from someone A. In the same field (posters, specifically) and B. someone who is still alive and trying to make a living.
I personally think that the idea of using clipart/found art/iconic/copyrighted material/whatever is to put it into a new context or give it a new meaning. If it's taken from a poster already in existence, in my book you've already failed. At least conceptually.
I tried talking about this topic a bit on Ewing's Eddie poster and I think I was taken the wrong way...
Interesting point (I think) because decorative border art type of stuff is considered by many to be "free for the taking". I think alot of people look at it as if we were discussing two different pictures in the same style picture frame...