I believe it;s a model 21? It's a 6.5 HP Briggs & Straton engine. From what I"ve read the no. doesn't make sense? Would the serial no. help. It's also a power driven model. Would anyone no the plug number. I think the man that fixed it put one in that's too long. As you know it may be too close to the cylinder head. Thanks Paul ps will send picture asap