Thanks, Chad. That's great information.
A little exploration around the internet reveals these are Blenko product #966, designed by Winslow Anderson in the early 1950s. The popular name seems to be "amoeba".
They were made in a vast variety of organic shapes and typical Blenko colours, as well as the white and clear combinations shown here. For a typical range see the 1960 Blenko catalog here
. They remained in production until well into the 1970s.
I'm still curious as to how
they were made: obviously by hand and presumably with the assistance of open molds.
They were inexpensive items then and now. The price in the 1960 catalog is $2.00 (presumably wholesale). There are several on offer on US ebay and other auction sites for not much more than that in real terms, mostly $20-30.