The ORA from Mullard was designed in 1922 as a development of the K valve and the redesign was prompted by the commencement of broadcasting.
This exhibit has much in common with ORA but one important difference, the envelope is etched with the words Exhibit Only and not the usual markings.
The ORA stands for Oscillator, Rectifier, and Amplifier - its planed usages. It is a bright emitter valve and had a 5 (amplification factor) of 8.5.
The etched letters of EXHIBIT ONLY on the envelope.
A better view of the lettering.
The grid wire is held between two supports and fixed in place with a thin wire running round the supports and grid winding. The development in ORA was to hold the grid wire in a pair of twisted wire supports. This method of construction was very labour intensive. In the end Mullard wound the grid onto a single support in much the same way as the original R type.
A close-up of the lower part of the grid showing the construction method.
The thin glass tube envelope is 25 mm in diameter, and excluding the B4 base pins, is 80 mm tall.
References: 1003 & 1004 Type ORA was first introduced in 1922.