Most are epicene as already stated (p. 246).
APPENDIX II
SPANISH COMMON NOUNS, IN ORDINARY USE, WHICH ARE USED ONLY IN THE PLURAL
Afueras (environs)
Aguaderas (frames to carry water)
Albricias (reward for good news—also used as interjection: joy! joy!!)
Andas (stretcher, also frame for carrying an image)
Calendas (calends)
Calzoncillos (drawers)
Carnestolendas (carnival)
Celos (jealously—"Celo"—zeal)
Hacer cosquillas (to tickle)
Despabilladoras (snuffers)
Enaguas (skirt)
Fauces (gullet)
Modales (manners)
Mientes—also Mente (the mind)
Parrillas (gridiron)
Puches (sort of fritters)
Tenazas (tongs, pincers)
Tijeras (scissors)
Tinieblas (utter darkness)
Víveres (victuals)
Zaragüelles (kind of breeches)
Note the following—
Á ciegas (blindly)
Á ojos cegarritas (blindly)
Á horcajadas (astride)
Á hurtadillas (on the sly)
Á sabiendas (knowingly)
De puntillas (on tiptoe)
Á tientas (groping)
De bóbilis bóbilis (without toil)
De bruces (on all fours)
En ayunas (fasting)
En volandas (in the air, off one's feet)
The following plurals, besides the ordinary, have also a different meaning from the singular—
El alfiler (pin) Los alfileres (pins, pin-money)
El día (day) Los días (days, birthday)
La esposa (wife) Las esposas (wives, handcuffs)
El grillo (the cricket) Los grilles (crickets, shackles)
La letra (letter) Las letras (letters—literary knowledge)
El padre (father) Los padres[212] (fathers, parents)
[Footnote 212: In the same manner all masculine plurals include also the plurals of both genders, as: Hermanos (brothers, also brothers and sisters).]
We make the following remarks on the plural of compound words.