Terry,

The Clark Y airfoil actually had a specific set of coordinates - heights of upper and lower surfaces listed by % distance back from the leading edge. I doubt many "true" Clark Y aifoils were ever used by modelers. Instead, just about any airfoil with a normal looking top surface curvature and flat bottom surface, from near the leading edge back, has been called a Clark Y-type airfoil. It was, probably still is, popular for free flight sport type models, and other uses where you basically only need lift when the model is right side up. (A "lifting" airfoil, not symmetrical.)

Pollywog airfoils' shape drops below a smooth outward-bulging curvature in the back third or so of the wing's chord. Most pollywog sections I've seen were symmetrical, usable for stunt. There's a story often heard that an early kit manufacturer changed the ribs in his kits to this shape so that he could print or die-cut more ribs on a sheet of balsa that fit into his kit boxes. If you've ever seen a Free Flight "undercambered" airfoil wing, you should get the idea...

Pollywog airfoils  are a bit tricky to cover, since the paper, or whatever fabric, shrinks, and will try to lift off the ribs where the curvature is below a straight line or outward curve.

Speed and racing CL models usually don't do much inverted or outside stuff, so they use "lifting' airfoils. Many full-size planes used "lifting' sections, so Scale and Carrier models often do too. You may still hear these called Clark Y-type...

Pollywog airfoils work well for stunters, but there's no real advantage over more normal sections that don't have that tendency for the covering to lift away from the ribs.