In the late 1970's, one hundred FQX_ wagons were targetted for conversion to FQF. The reason for the coding was to reduce the number of "X" wagons in service. Several vehicle classes were affected.
The number group selected for conversion was FQX 501 - 600.
The vehicles were all to the same design. The main differences between some were the additional "lashing rails" added when vehicles were serviced.
The numbering and conversions did not appear to be strict as several FQX wagons in the 500's could be seen for many years and and FQF 637 was noted.
The FQF wagons were recoded to VQCY in the 1979 recode.
The VQCY wagons were reclassed to VQCX between 1987 and 1990.