"Note there is no external seam."
I'm thinking the seam is just under the edge of the foot of the grape part, and that the edges of the large bubble were folded under there and attached (see bad drawing, blame it on my mouse). It's very hard to see from the photos, though. The bottom would have had to be open to allow air to escape from the grapes as it was cooling.
If the grapes were put in through the top that would mean a lot of glass to close in to make such a long, narrow neck.