As Phillip mentioned the disk guards are slightly more common on more rustic examples. I have two, one put together in the 19th c. with an older blade. The other is likely older, and the guard was made at the same time as the blade. The 19th c. version does not have a tapering handle like most jian, but was put together like a dao with a jian blade, while the older one has a tapering handle more typical of jian.
Even among rustic weapons the disk guards are not common. I would estimate their frequency at less than one in a hundred. Interestingly, dao with jian type guards are fairly common. On some, such as the dadao, it is somewhat unusual to find a disk guard.
I suspect that the extra protection of a disk guard was not worth the pain of having it digging into the person carrying it. So we rarely see it on dadao carried on the back, rarely on jian carried around as status items, and more commonly on rustic jian that were probably not carried on a regular basis.
The disk guards are almost ubiquitous on military dao where presumably the extra protection was worth the added effort.
Josh
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