The ARP35 is the UK Army, pre-WWII, name for the EF50. It is clearly a utility valve with its plain aluminium screening can devoid of the familiar red point.
The valve is a high impedance variable μ pentode and was a workhorse of receivers for TV, radio and especially war-time radar.
The wide glass tube envelope is 32 mm in diameter and, excluding the B9G base pins, is 60 mm tall.
References: Data-sheet & 1043.