Just thought I would sing the praises of this particular rubber grommet which is a great replacement for the hardened and brittle feet on my J5.  I bought a bag of 8 for just $14 off e-bay.  It took about 20 minutes to remove the old feet and then reinstall the new feet using soapy water and a screwdriver to wedge them in (which did take a little effort". They work perfectly...no slippage at all. 
If you order them, make sure you examine the pictures and size carefully as they offer a large variety of grommets and feet. 
e-bay listing:  5/16" Tall Rubber Feet Push In Ridged Fits 1/2" Hole & 3/16" Panel 8 per Pack | eBay

Here is a web posting that inspired me to get these rubber feet: Building Custom Desks: Typewriter - Adler J5, 1974, Replacement Feet (buildingdesks.blogspot.com)