Chingolo or Palomita (chuck tender in the U.S.), is a cut of beef that consists of the entire supraspinatus muscle which lies laterally to the shoulder blade bone. Contrary to what you may be thinking about a cut with tender as part of its name, chuck tender is anything but tender. Slow cooking, as with many other cuts from the chuck, is needed in order to turn that toughness into tenderness. For this cut, braising or stewing is typically the cooking method of choice due to the low amount of fat. However, when properly cooked on a parrilla you will be rewarded with a delicious chunk of meat full of rich beefy flavor.
Chingolo is typically sold either in whole form (some may be offered butterflied) or as steaks. For parrilla cooking, I recommend choosing a whole cut that leans toward the heavier side of the weight spectrum. At least one kilo (2.2 lbs). Since chingolo is a whole single muscle, it is surrounded in part by a thick membrane of connective tissue (silver skin). Butchers usually trim most of that off but if not, ask them to do so unless you are skilled at such a task.
Follow the same instructions for aguja. If the chingolo has a very large girth and looks like it may need way more time than any other cuts of meat you are cooking, butterfly it down the length.