A long time ago, the flexible track used to sold in Steel, Brass or Nickle Silver options. The codes were:~
SL100S Steel (S)
SL100B Brass (B)
SL100X Nickle Silver (NS)
While flexible track was available as above, points and crossings were just NS hence the 'X'.
After Brass and Steel were dropped the 'X' part of the poduct code was dropped.
Graham