parts; helical gear, toro, self propelled lawnmower

Inside the the toro self propelled mower's gear box are two main gears, a worm gear and driven helical gear. Every site I go to lists the worm gear, but none the helical. Model is 16775,sn 9007923. Any idea why this particular part would not be listed? I'll call around on Monday but hope maybe someone knows why... and maybe where I can find one. The whole gearbox is $250 or so. Thanks
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