Pantone used to produce a guide that had color specifications with "2X" after suffix, e.g. PANTONE 185 C 2X.
2X = double impression, usually referred to as a 'dry trap', where a color is printed, dried back, and then overprinted a second time to produce a more intense color.
These 2X colors were deleted from the PANTONE MATCHING SYSTEM products back in 2000. The reason for this is because they were rarely used, and caused unnecessary confusion when they were specified.
While it is still technically possible to print double-impression colors, Pantone does not currently provide examples, and thus any existing guides or books would no longer be considered color-accurate.