Petite Sirloin Steak
This cut seems to vary in interpretation. Some vendors claim theirs are from the Top Sirloin, but it seems more official that they are one of the cuts from the Sirloin Tip, or Knuckle (See Round). This cut has very good flavor and is quite moderately tough.
The photo specimen was 8 inches long, 3 inches wide and 7/8 inch thick, weighing 8.9 ounces. It was stupidly priced at 2013 US $9.99 / pound, but if you bought one tray you got another "free", so the effective price was about $5.00 / pound.
More on Cuts of Beef.
Cooking: This is another cut that should be grilled or broiled over very high heat so it is almost chared on the outside and quite rare on the inside, and then is sliced thin crosswise. I have also sliced these in half horizontally and pan fried them successfully. They were a little chewy, but not at all disturbingly so.