The 1821 is a Philips full wave rectifier on a B4 base with a brown composition 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.
Unlike our other examples of this Type this one is 23 mm shorter and a diameter some 6 mm smaller. The suggestion is that this is a later product - possibly post war.