The CT-15 is a general purpose triode with a 2.0 V filament. The anode voltage range is 30-120 Volts. Electrically the CT-15 is identical to the CT-08. The only difference is the filament - CT-08 runs at 3.0 V at 0.08 A.
The silvering of the envelope makes inspecting the electrodes difficult.
A close-up of the cylindrical anode and helical grid. CT-25+ shows a double cylindrical anode.
The balloon envelope is 34 mm in diameter, and excluding the B4 base pins is 91 mm tall.
References: 1003 & 1043. Type CT-15 was first introduced in 1925.