It turns out that the slightly green hue visible in the top of the piece when it is placed directly in the sun is a classic feature of Vaseline Glass. I found lots of examples of this glass on-line and in my collection of books on glass. The difference is that all of the examples I found were pressed or cut glass, where this piece is obviously hand blown.
I have been buying and using 'heritage' glass for nearly two years and sometimes question whether some of the pieces I have had cut/fused have been little pieces of history that should, perhaps, be protected. So far I have been able assuage my conscious by telling myself that I am offering them a new life glistening in a window (instead of gathering dust in the back of a cupboard). For now, I am holding onto this piece while I do a little more research. It could be interesting to have a feature piece in a panel that changes from yellow to green in the sunlight ....