The U5 has the distinction of being the first full wave rectifier. The U5 was introduced by the Marconi-Osram Valve company in 1926.
The paper label with the Osram name has almost been lost and only fragments remain.
From the picture it can be seen that each filament is an inverted V. It looks as if the filaments are connected in series but the two outer connections are joined at the base pin. Each anode cavity is a flattened cylinder.
The coated ribbon filament and the tensioning spring supporting the top of the loop. Note the use of the glass rod to support the tensioning springs, this construction pre-dates the introduction of mica shhet supports.
The balloon envelope is 56 mm in diameter and, excluding the B4 base pins, is 109 mm tall.
References: 1005 & 1043. Type U5 was first introduced in 1926.