This exhibit is a copy of the M-OV Type S215 that was (we think) the very first 'upright' screen-grid valve to reach the British market.
The back of the externally metalised envelope is marked as Foreign even though the front proudly proclaims Triotron as Austrian. The top cap has lost its insulated nut.
Another Triotron S215 complete with top cap as sold.
The close wound screen grid. Although the electrodes are mainly obscured, but the construction is the same as can be seen in PM12.
The balloon envelope is 50 mm in diameter and, excluding the B4 base pins, is 124 mm tall.
References: 1005 Type S215 was first introduced in 1929. See also1929 adverts.