What (specifically) do these eight letters have in common, and with that in mind, what's the only other letter of the alphabet that should appear in this list?

O, P, S, U, V, W, X, Z

HERE is the answer.

THE ANSWER:

"C"

Each of these letters appear virtually identical when printed in upper or lower case:  Cc, Oo, Pp, Ss, Uu, Vv, Ww, Xx, Zz

Six other letters are close, but not quite as similar:  Ff, Ii, Jj, Kk, Mm, Yy

The remainder of the letters are dissimilar:  Aa, Bb, Dd, Ee, Gg, Hh, Ll, Nn, Qq, Rr, Tt