Yes, I did asked questions there before reading the rules... Sincere apologies.
It was an older thread, so started a new one.
It seems that the only part that's missing is screw 268. I found a picture of it in the list of the parts, also on Smithsonian website a book of screws and nuts, but no measurements.
I have some spare screws and one will fit in the hole in question, but it's much too short...
I also tried to measure it, (tried with my caliper, but it's metric so it is showing 4.42mm for it's circumference). I believe Singer used to make it's own screws, they are harder to find now. (My Dad is a mechanical engineer, he told me the threading is most likely in a different angle too then metric screws)
I also find a great source in another thread here about Singer screws (this forum is amazing ). A file was added (I can't open it for some reason, perhaps it's been removed?) about Keystone having a catalogue of Singer screw measurements...
I'll email them and see weather they can help. Also got in touch with Helen Howes in the UK, she has sewing machine parts and was able to help me out before...
The machine cleaned up beautifully, and it's turning pretty smoothly. It is one of 200 made in 1951. I am sure it's sewing a beautiful stitch. I can't give up on it just yet...
Thank you again for your help!
Eszter