In the Kodak Dataguide "Color Separation and Masking" they give only one method of masking as follows:
Light Source ------> /Original Emulsion/Base/// Diffusion Sheet .003" max optional ////Emulsion of print material/Base/
The optional iffusion sheet is recommended for unsharp masks, but is omitted for normal masks. This is from page 39 of the manual. It goes on to show that the masks are used emulsion to base with the original closest to the lens.