The 5Y3GT is traditionally a directly heated full wave rectifier, with inverted v's of coated tape as filaments. However, this implementation looks the same as the GZ30 in terms of the same getter holder, glass foot construction and is clearly indirectly heated. This looks to be from the 1950s or later.
The maximum reservoir capacitance for the 5Y3GT is 32 µF and minimum series resistance is 100 Ω, The GZ30: 50 µF, series resistance 380 Ω
Still the reluctance of the UK to accept imports as of equal value to British made. Foreign is a pejorative term.
The twin anodes with the same getter holder as used on the Marconi GZ30.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 69 mm tall.
References: Data-sheet, 3002 & 1040. Type 5Y3GT was first introduced in 1936.