Vespa Battery box

I have a 1977 Vespa Primavera VMA2T that has been off road in storage for ten years. It has a black box with a battery pack of five small similar to C sized batteries set up in series with an internal fuse, capacitor,etc. It has four exterior leads... Pilot lamp, directional, earth and generator.The batteries are corroded and need replacement if not the whole unit. There is no other battery present but one may have been removed though doubtful. 

I am uncertain if this is the original set up or has been modified. Any info is appreciated and recomendations as to where to purchase this and other parts including a more model specific parts/ repair manual if available.. I downloaded a general Service Station Manual which doesn't show this part or setup.

Thanks for any help.

Jay in Charleston South Carolina US.