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. For service valves it is not usual to have the manufacturers name on the envelope in plain language.
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: Datasheet & 1043.