Yogiboy and geoffkait are correct. I've sometimes referred to these records as "Dynaflaps" because they were so thin. But some were very good sounding - like ARL 2-0731 - Schubert Piano Trios with Rubenstein, Szeryng and Fournier. Still play that one once or twice a year.
Dynagroove was not a recording process RCA Records used, but rather an LP pressing technology, and a real, real bad one. Some RCA titles were pressed in a few different versions of RCA L:P's: Shaded Dog, Black Label, Red Label, and finally in the late-60's as Dynagroove LP's.
+1 bdp24! RCA's failed attempt to improve the sound by altering the EQ during the process of cutting the master. None of the resulting LPs made from this method sound good! Worthless junk, IMO! The record jackets stated on the fronts "Dynagroove".
After Dynagroove failed to increase sales, RCA came up with another dubious scheme: Dynaflex. Pressing LPs with HALF (or less) the amount of vinyl! Supposedly to prevent warpage due to shipping! These actually can sound quite decent, as long as they remain without warpage.