6-26-12 There are two different t25 cutting heads. The curved shaft has a Right hand thread and the straight has a t25 left hand thread, because of the gear box on the straight shaft 128ld, that turns counter clock wise, where the 128cd turns clockwise. They are two complete different arbor sizes, one LH and one RH.
06-26-12 I just bought the bottom shaft from partstree.com You just can't buy the dust cap. You get part 16 the lower shaft, which includes part 21, part 22 the dust cap and a new trimmer head. It costs 20.95 plus shipping. I also bought the part 17 the kit shield to fit on my 128ld straight shaft. You maybe call first, but this works for my 2010 model.
Click on this web site: http://www.partstree.com/parts/?lc=husqvarna&mn=128+CD+%28USA%2C+Canada%29+%282010-03%29&dn=46540001