This is really interesting ... I had noticed that some feet weighed more than others. For our family's 301, 401, & 500 I noticed that a few of the new 1/4" feet have been less or more than 1/4" and this is because of the way their shanks position them. Supposedly they were made for slant shank machines, but they aren't accurately positioned.
Okay Joe, now let me reveal my stupidity.... I am confused about why presser feet need cushions. When my sewing machine is at rest I have always kept the presser foot in the up position. Is this habit a No-No? Should it be kept in the down position with a cloth cushion beneath it?