Reynolds do a mind boggling range of tubing with different characteristics for different applications. Don't read too much into the series number as that only denotes a particular production method, not Its awesomeness.
OK, let me put it another way, the bigger the number, the more expensive it is.
I know there were several versions of some of the "numbers", for example 531ST (Special Tourist) and 531C (Competition) which indicated the particular alloy and I'm guessing that versions denoted that tubing was the same basic material, but different guages/thicknesses and perhaps different cross sections and butting to give the finished frame different characteristics - lightness, stiffness, durability etc. In any case, I generally find that Reynolds numbering system is a bit less cryptic than some of the names other manufacturers use which make it a little unclear as to where in the range a particular tube set is.