So excited for you. I understand the feeling of finding a piece of family history. I have my Grandmothers 66 that she got the year my mother was born - 1928. She wanted to sew dresses for her little girl and my Grandfather bought it for her. Unfortunately she was widowed 3 years later. Even having to work to raise 2 kids during the depression, that was the only thing she didn't sell. Eventually she gave it to my Mom when she had kids - probably in the late 40s. Then about 1960, when I was born, she went and bought a 401, and put it in the same cabinet. She told me she didn't know what happened to the original "Black Singer" machine.
Then I was at my cousin's house a couple of years ago. I asked to see the Featherweight she inherited from her Mother. Her husband mentioned the "other 3 machines in the garage" and my cousin said she had Grandma's machine too, did I want it? I figured out that when my Mom bought the 401, the shop put the 66 in a case (it's a 50s maroon-and-grasscloth case) and she gave it back to my Grandma. I don't think my Grandma ever used it again but it was stored with her stuff that my Uncle kept after she died. Then my Uncle died and my cousin cleaned out his place. It's rusty, frozen, and had no manual or attachments (I've since replaced them), but it's history is with this family. Since the 401 and cabinet are promised to me, one day it will go back into the original cabinet. I even found one of the rare 3-drawer Singer side cabinet to match the #40 cabinet (I'll have to refinish them together as they are both in bad shape), But yeah, Grandma's machine.