20 series has a V shaped belt. 30 series has a belt with one flat side and one vertical side. 40 series is much bigger, more heavy duty, handles more hp V belt style. It also costs a lot more money.
The 20 and 30 series driven clutches are different as well. You Can't mix and match 20 series to 30 series. For use on a Rupp, due to the jack shaft and where the engine is bolted in, you need a 30 series with a 6" driven clutch with a 5/8" bore.
20 series and 30 series both cost about the same.
Avoid cheap belts. The China belts work but wear out fast. Get a genuine Comet belt. There is another Good brand , Bando. I haven't used any but read they are good premium belts.
The video below should answer some questions.
Danford1
PS. I was wrong in my other post. Rupp's need 30 series clutches, not 20 series.
[video=youtube;GvfkpWxxHuA]https://www.youtube.com/watch?v=GvfkpWxxHuA[/video]