Original Hario replacement upper bowl (BU-TCA-2) for the Coffee Syphon Technica TCA-2, the 2 cup model. Made in Japan from heat-resistant borosilicate glass, it fits the lower globe exactly like the part your syphon shipped with, restoring the vacuum seal that syphon brewing depends on.
The upper bowl is the most exposed piece of a syphon, so a spare is worth keeping at hand. Compatible with the TCA-2 only: the TCA-3 and TCA-5 use larger bowls of their own.