From the description, it sounds like a Top Sirloin Butt. I've never smoked one but a friend has and I recall it as being quite tasty.
Many chains such as Applebees & Chilis use this cut for sirloin steaks. It takes a bit of practice to learn the ropes as per fabrication but the end result is about 1 lb of burger/chili meat, a 6 # center piece which is what the chains use...can be cut to faux Filet Mignon thick cuts or longer, thinner steaks. There's a triangular piece of meat just under the fat cap which is similar to Tri-Tip...a fabulous grilling cut that is best sliced a la flank steak.
For whole smoking I'd recommend Montreal Steal seasoning and have at it. It's best medium rare and you'll get some carry over. I'd pull it at 140 tops.
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