i believe for the R kits, it is a standard part, for a certain numbr of kits atleast.
to the original mtx4 kit, they are option parts.
the R came stock with option parts, thats why its the R
and also an fyi, my mtx4 manual had a typo on the side pulley assembly page, it states the pulleys are 23t and 24t, instead of 24t, 25t. i have had my kit for about 2 years, and i just noticed this