Some articles on keset:

Parashah - Parashot in Ketuvim - Esther
... In the nineteenth century, Rabbi Shlomo Ganzfried published a manual for scribes called Keset HaSofer, in which he follows the rule that all parashot in Esther are closed {S} (K ... added that "some authorities validate" it (Keset HaSofer 285) ... have been listed in the chart below under at "OH" under Keset HaSofer, and they are very similar to what is found in the Tiberian masoretic codices ...