In 1981 there was the " Extra Affordable " models .. C500S, C512S & C537S.
Not sure about differences between the C537 and C537S models.
The C512 had flake and the C512S did not, not sure of any other changes.
The C500S could be the yellow C500 that I have found about 18 of.
All the yellow C500's have CV style seats in pleated yellow vinyl, most have blue carpet and a different style steering wheel.
They don't have a 18 gal. bow mounted fuel tank like the standard C500 had.