I think the push/pull tone pot is pretty recent (2008?). They stopped making 4001's in 1982 or so. I had a 1979 jetglo, really thin, fast neck, and very trebly bridge pup. I think the 4003 has better electronics and hotter pups, always had a little trouble with thin sounding bridge pup on the 4001, though it did nail a Squire/Lee sound pretty well. The older basses were effectively unique, with a lot of variation in dimensions, neck construction, fingerboard material, etc. The new basses are all CNC machined, so dimensionally identical. Even post CNC, most Ricks seem to have more individual character than a lot of mass produced guitars, probably due to all the fussy handwork needed to get that incredible finish.