Below is a page from one of my old notebooks, giving some of the evidence I was considering in 1989 for the decipherment of the tza syllable sign, a version of which is shown at right. The reasoning was pretty tentative back then, but it was bolstered in my own mind by considering more examples of the sign in the spellings of tzu-tza-ja and TZUTZ-tza-ja, for the passive for of the verb tzuhtzaj, “it is finished” (See Stuart 2001). Shortly after this page was jotted down I came across completive forms spelled 2tzu-ji-ya (with the tzu syllable doubled) for tzuhtz(a)jiiy, “it was finished,” which made the tza reading fairly certain.
The tza syllable sign is not a very common sign. It crops up in only a handful of other spellings, such as tza-ka or tza-ku for tzak, “to conjure, manifest (a god)” — a reading otherwise familar for the “fish-in-hand” logogram (TZAK).
Stuart, David. 2001. A Reading of the “Completion Hand” as TZUTZ. Research Reports on Ancient Maya Writing, No. 49. Center for Maya Research, Washington, D.C.