According to the 38-39 Early Ford V-8 Club restoration book, "Some shock absorbers were filled with a blycerin-based shock absorber fluid, M-1046-B, and should havea flat top filler plug, 353054-S. Some were filled with a mineral oil based fluid, M-4633, and should have a tip on the filler plug, 35068-S. In 1939 a sostor oil based fluid was added.... Service Bulletins indicate it is important NOT to mix different types of fluid in the same shock absorber."
I think the latter just recommends draining the old fluid out before adding any new fluid.