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Welcome to Just Answer and thanks for your question. Necchi BF came in a number of different models (Mira, Nova, Supernova). You can download the manual for whichever model you have from this site for $10 http://www.sewingmanuals.com/Sewing_Machine_Manuals/Necchi.htm
You can also call Allyn and they can obtain parts and manuals for you. http://www.allynint.com/
Manuals for these vintage machines are not available free online. Most of them have been out of print for at least 30 years. You are lucky to found an old vintage BF. Only a technician you deals with these old models, or the proper manual, can determine what all the buttons are for. I did see that Allyn had a cording foot for it though. You can call them for pricing.