Welcome.
I'm not really familiar with industrial machines, but I do like to research to try to help. How did you figure out how to thread it, without a manual?
From what I've read about industrial machines, they usually take a different system of needle than the domestic sewing machines. According to
http://ismacs.net/singer_sewing_mach...s-400-499.html this machine uses a 88x1 needle which has a round shank.
Additionally, some/most? use left twist thread, especially for the top thread. I found a manual at
https://www.tias.com/download---pdf-...pdf-33769.html and
https://www.manualslib.com/manual/36...ger-400w1.html There is also a parts list at
http://www.supsew.com/download/Singe...er%20400W1.pdf
What are you planning on sewing with this machine?
Since I'm in the middle of the US, I can't help with sewing machine shops in your area.
Janey - Neat people never make the exciting discoveries I do.