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. Originally the 1821 was made with a balloon envelope.
The filament is an oxide coated tape that forma an inverted V and is tensioned with a spring above the top mica.
The central rods are only there to support the filament tension springs.
The classic envelope is 44 mm in diameter and, excluding the B4 base pins, is 100 mm tall.
Reference: 1043. Type 1821 was first introduced in 1930.