Well the 10518 is the same mower as the 10515 and 10520 except the 10518 has the plastic carb and the 10515 and 10520 have the metal choke only walbro carb. They are the exact same mower except the plastic carb puts out 5 hp and the metal walbro puts out 4.5. I prefer the metal walbro over the plastic carb because you can adjust the wabro and you can't on the plastic modular carb. Another thing to check when you change the belt is that the set screw on the pulley hasn't sheared causing the pulley to just spin on the crank shaft. I saw one last year that had the whole sp system disabled and all that was wrong was the set screw and a belt. You have a keeper with the mower and the gold F's are the best of all the gold series. However i am seeing several of these later model F's have a problem with throwing rods so keep the engine governed correctly and sufficient lubrication. I am getting these mowers used and i don't want to start a debate about lean mixtures, but i would keep it very close to 32:1 mixture. If it were my machine and i was using the newer synthetic oils I believe you could get away with 50:1. Like i said I'm not trying to start a debate, just telling what i would do if it were my machine and i wanted to make sure i kept it running for many more years. I guess the way i look at it is that lawn boys were designed to run 32:1 and that is where mine will aways run. Congrats on the score and i hope this sheds some light on the model differences.