The 1821 is a Philips full wave rectifier on a B4 base. The UU5 is given as an equivalent but has twice the rating, so the UU5 may replace the 1821 but not the other way round. Later examples of the 1821 were made with the classic or shouldered envelope.
The gold paint on the dome gives the identification information. Mullard was the Philips subsidiary in the UK and gold paint was their speciality.
The anodes are hollow towers with oxide coated ribbon filaments within. The anodes are fixed securely to the glass rods as are the filament tension springs.
The support structure is a mass of spot welds. Not a cheap manufacturing option.
The pinch and filament connections. The filaments run in parallel.
The balloon envelope is 52 mm in diameter and, excluding the B4 base pins, is 110 mm tall.
Reference: 1043. Type 1821 was first introduced in 1930. See also1930 adverts.