I’ve finally found a reference as to the difference between strips & whole belly clams. They are, indeed, two different clams (hard vs. soft shell) – strips are not simply the same clam with the belly removed.
“The United States is the only source for surf clam, which is too big and too coarse to be eaten whole like other clams. Instead, they are sold processed, rather than live, in fresh, frozen, and canned products such as clam strips, minced clams, stuffed clams, chowders, and broth.”