Marble's is still alive and well although the winds are changing. I just received one of their Plainsman models in the new mystery steel and will be comparing it alongside my old model in 52100. Should have some results posted by this weekend.
It is true that they have limited their standard offering to four handle materials (leather, wood Game Getter, bone, molded rubber). They will also continue to do limited custom runs (300 or so per run) using a variety of natural materials.
BTW, for those interested, the tang stamp now just reads "Marble's" with no reference to Gladstone, MI.