In a midrashic analysis of the source for the minimum acceptable height of a sukka (ten tefahim), the Gemara looks to the height of the aron – the ark containing the luhot ha-berit (the tablets of the covenant) – which was ten tefahim high. It is clear from the Biblical description that the aron was nine tefahim high; the additional tefah was the height of the kaporet that covered the aron.
Our Gemara seeks to find a source for the fact that the kaporet was one tefah high, which it derives from a comparison of the kaporet – which does not have a specific size mentioned in the Torah – to other utensils used in the Mishkan.
One of the High Priest’s accoutrements in the Temple was the tzitz, the golden plate worn as part of his bigdei kehunah (priestly garments), but the Gemara says that it cannot be used as a source for the size of the kaporet because, as part of the High Priest’s uniform, it was not one considered to be one of the utensils of the Mishkan.
We know details about the appearance of the tzitz thanks to Rabbi Eliezer son of Rabbi Yosei, who testified that he had the opportunity to examine it during a visit to Rome. He describes it as having the words Kodesh la-Shem on a single line, as opposed to the opinions in the baraita that describes it as having the word Kodesh on the bottom and la-Shem on the top.
Rabbi Eliezer was the son of the tanna Rabbi Yosei ben Halafta and lived in the last generation before the redaction of the Mishna by Rabbi Yehuda HaNasi. Rabbi Eliezer was, apparently, the greatest of Rabbi Yosei’s five sons and, already during his father’s lifetime, he was recognized and honored by his generation.
During a difficult period for the Jews, Rabbi Eliezer was, along with Rabbi Shimon bar Yohai, part of a delegation to Rome that tried to get decrees against the Jews rescinded. While in Rome they were miraculously given the opportunity to heal the Caesar’s daughter, who had fallen ill. After successfully healing her, they were offered the chance to examine the Caesar’s coffers, which included the spoils of the Roman victory and sacking of the Land of Israel and the Temple. Rabbi Eliezer’s examination of the Temple remains allowed him to return to the Sages with information about a number of the utensils from the mikdash, including the parokhet, the tzitz, etc.