[CR] Legnano bb question

Noodling around ebay I came across an auction of a repainted 1960's Legnano. The third to last photo is of the bottom bracket and it shows a nut and bolt on the underside of the drive side. I was wondering if someone could tell me what it is for?

http://tinyurl.com/4fzmmnx or item 120682555583

Thanks!

Len Diamond

Ridgewood, New Jersey USA