The two-groove main pulley and the second idler are for reverse. It runs of the camshaft output shaft. You don't have this because someone replaced the engine. I believe original engine was 80000 series 3hp. Your tiller is from early 1970's; replacement engine was from late 1980's to mid 1990's. You should be able to get model number from label on frame next to drag stake. Probably 902002, but could be one of dozens.

the label is there but no numbers are visible. but i'll look into the 902002 series.

I have a similar tiller my is Mod # 902985, It is missing the engine, but was able to fine out which one came on it from Ariens, they were very helpful. My question is with out the engine I don't know how the two belts go, I am restoring it and need the belt diagram or a picture.
Thanks Lenny.

Hi Lenny I have the same tiller but can not read the engine number. I believe its the original engine . What engine came with it ? Do you have an engine number. I don't have the oil dipstick and can't find a replacdment.

I just got a 3hrp it is from 1974 is a 80000 series looks original

Lenny there is a belt diagram on a site for ariens 3 hrp tillers I just saw it