Capacitors with extra legs are not uncommon. The extra legs serve TWO purposes. The first is to "key" the capacitor such that it only fits into the circuit one wya. When working with this level of energy density it is a good idea to do this. Second, they serve as additional mechanical connections to the board in order to stabilize and support the capacitor. These capacitors are large and heavy; note that it is 64mm (2.44 inches) long and 35mm (1.38mm) wide and weights 67g. This will have significant force in areas subject to vibration, often enough to either pull out the leads, break the solder joint or worse pull the cap right out of the board. As such, it is actually quite common to find 3 and 4 legged capacitors in larger can sizes. I have some Mallory caps in an old (but VERY nice) HP Precision 6114A power supply that have three and four legs. They are quite large (the largest is roughly the diameter of a can of red bull.