This valve dates from about 1926 and was made by M-OV. It is believed to be the same as an NR14 but fitted with a B4 base.
Externally it looks like a Type DE3 but the internal details are different. The Admiralty had there own ideas about valve design and it is believed that there was no direct commercial equivalent or prototype. The CV designation corresponding to NR17 is CV1155. The μ of the valve was 7.
The NR in the designation stands for Naval Receiving.
When seen from the rear, the anode becomes visible against the silver of the magnesium deposited on the inside of the envelope when the getter was fired during manufacture.
The detail photograph has been enhanced to show both the grid and the spiral filament at its centre.
The filament is a thoriated tungsten dull emitter.
The thin glass tube envelope is 24 mm in diameter and, excluding the B4 base pins, is 80 mm tall.
References: Private communication. Type NR17 was first introduced in 1926.