The ARP35 is the UK Army, pre-WWII, name for the EF50.
Valves supplied to the services are usually left in plain aluminium cans without the maker's name. This exhibit is clearly a Mullard EF50 with ARP35 printed in place of the commercial designation.
The valve is a high impedance variable μ pentode and was a workhorse of receivers for TV, radio and especially war-time radar.
The reverse is unmarked.
The wide glass tube envelope is 33 mm in diameter and, excluding the B9G base pins, is 58 mm tall.
References: Data-sheet & 1043. Type ARP35 was first introduced in 1938.