Unlike the various muscle meats and organs held at low heat, on full display, my chosen off-cuts were secreted under a damp towel. When they appeared, thoroughly chopped, I didn't recognize them by their looks, only by their gelatinous texture. Tacos de ojo (eye; 16 pesos each, about 80 cents at the time).
Los Cocuyos
Calle de BolĂvar 56, Centro, Mexico City
+52 55 5518 4231
(From a March 2017 visit)