He also referred to it as Prime Brisket Tip (though I don't think they are from the brisket at all).  These are triangular pieces of meat, I got 2 of them, each about 2 lbs.   

When I checked again with another butcher at the same shop, he called it prime bottom sirloin.  This same butcher say that around his house they call them "football roasts" because the meat plumps up when you cook them.

By the way, although this cut can be had inexpensively in other parts of the country, these are not cheap cuts of meat around here... the 4lbs cost me $31.00... so it pays to do them justice

I sprinkled JJ's rub on the two tri-tips and let them sit in the refrigerator for a couple of hours.

JJ's rub is something that we make up in triple quantities and keep on hand.  The recipe is found here.

I cranked the #7 up to about 600 degrees, seared each side for about 4 minutes then closed the air flow enough to graduate down to 350 degrees. I tossed in a hunk of hickory and let them cook until the internal temp was 135-140 degrees. Took them off and let them rest for 15 minutes under some foil.

I carved thinly across the grain, and served with some creamed spinach and rice pilaf that my bride prepared.

This meal was a hit. Oh, and since some potential company didn't arrive, we only made it through one of the roasts.

My wife said it was more like a roast beef than like a steak, it had a nice texture, and beautiful color. My older son liked the pronounced hickory flavor (he guessed accurately at the wood I used).

The next day, we sliced up the remaining roast and made fajitas.. another winner.