It's a mains filter. A simple LC network in line with a mains plug and socket. There's an X-class capacitor on the output side, which had obviously had enough. The stench was horrible. On with some gloves, and clean the thing up!
A new capacitor was soldered in, and the filter reassembled.
These caps have a hard life. It's difficult to date them, as the same Rifa capacitor is still available today.
Apologies for the appalling photo. I've got a new SLR camera on the way, which will hopefully improve matters.