This thread threw me for a loop. As a kid in the 1950s, I remember hearing the term isinglass refer to sheets of the mineral mica as used in stoves for a viewport to be able to see if the fire was burning.
On searching the web for isinglass, I found that it also refers to a gelatin made from the swim bladders of sturgeons that is used in making beer.
I cannot fathom why one would apply 303 protectant to either, but to each his own.
Paul
ps. Why would I ever want essence of sturgeon swim bladder in my IPA?