OK....my information from you and others suggests that there was cork on the front frame that holds the lens board. No information yet about any material on the front "slider"....Doug, Daniel, and any others....it was suggested, and I will follow such, that I purchase some very inexpensive thin sheets of cork with a self adhesive backing. Thin strips affixed to the sides of the front frame should supply(?) the required friction. The problems to be solved? Well, the front frame would slip and not easily "tighten" when the knobs were tightened against the frame. Very annoying to have to deal with a sliding frame that also would not always stay aligned correctly i.e., one side would be slightly higher than the other. The friction without the cork was usually enough for axis tilt....but not enough when carrying out front rise and fall.
BTW-Do either of you have problems with manually raising the frame for front rise....not talking about the vernier screw which makes small corrections easily. Every once in a while one has to raise the front frame more than anticipated-usually in taking photos of barns, etc., etc. With a heavy lens sometimes it is a bit of a struggle to push the frame up with my fingers! Hopefully the cork will keep the frame aligned so that raising the frame will be a bit easier.
Thanks for the help.