The simple rule is red numbers on the foot are new style machines, black numbers on the foot are legacy; these older models are identified by cast aluminium casings.
In 1998 Bernina introduced the next generation of sewing machines; at this time, they laid the foundation for future innovations, many of which we see on the machines today. The most significant change was the angle that the presser foot attached to the machine, this allowed space for the semi-automatic needle threader. The maximum width of the stitch was also increased to 5.5mm on CB Models.
At the same time, Bernina fully introduced models with 9mm stitch width capeability to the range. These feet all have a 'C' after the number eg 20C.
In 2012 the introduction of the Dual Feed models arrived. These feet have a 'D' after the number eg 20D.