The ER21A is a small thyratron or gas filled relay.
The cylinder forms one main electrode. Above the glass foot is a substantial ceramic insulator.
The second main electrode is a flat plate with a small rectangular portion pressed out at 90 degrees. This small flap is close to the third element. The trigger electrode is a plain wire that ends a short distance away from the tip of the flap.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 46 mm tall.
Updated December11, 2013.