J10 Sheaves and Belts

[rgleason]

Ametric.com Metric Poly-V Ribbed - J Profile

Ametric.com American Standard ANSI Poly-V Ribbed - J Profile

Perhaps someone understands how these designations work.
I can't find J-10 in the American Standard in the sizes needed, but can find J-12.

The Metric link does have J Profile with 12 Grooves, would that work with a J-10 Belt or does it need the shoulder?

I note that both links show the groove details for both J type belts and L type belts. The metric pdf does have L-10 10 groove pulleys but that has much bigger groove dimensions than the J type. (See the sections detailing this in the pdf.)

The other question I have is which material should I be looking at?
Cast Iron or Aluminum Sheaves?

I am trying to find sheaves that will work with a custom install to fit the 28SI.