It's just behind the scenes notation for different generations of Westfield.
The very first body design was what became the Pre Lit, this was a copy to a greater extent of the Caterham Seven.
After a legal case saw Westfield being forced to make the body style unique to Westfield, they released what we now know of as Kit Bodywork. It wouldn't really be known as this at the time though, it was just the body style offered.
Around 2000, give or take a year or two, (like much around Westfields, exact timelines are tricky, as there can be overlaps) the bodywork received a subtle re-style this is what we know as the ZK bodywork. In everyday use, it was still just the bodywork, for the first few years, but the ZK suffix was important to be aware of, as the nose, bonnet and scuttle were different to the previous Kit bodywork and couldn't be mixed from an aesthetic point of view.
In the early mid 2000's the FW body shape was launched, as this was to be sold along side the "traditional standard" body shape, to avoid confusion, it became much more common to refer to the two as FW and ZK styles.
There have been some sub variations to the body over the years, and chassis developments have not run neatly alongside body developments, each has been at their own pace. Plus parts are often retrofittable. So it would have been much harder for the factory to simply name them Westfield mk 1, mk 2, mk 3 and so on.