I also have a German Singer 431G. Haven't been able to find a Service Manual
for it. The guts are probably very similar to the US Singer 401A, you may be able to refer to that.
You can download the very hard-to-find technical Singer 401A SERVICE MANUAL == FOR FREE==
(in the back
of the 301A Manual - then scroll down to page 125):
I'm looking for help with Chainstitching. Do you have those instructions in your 431G User Manual? Also looking for the PART NUMBER of the 431 Chainstitch Throat Plate.
On AMERICA Online, I am PattBHS66.
Contact me and we can talk.
Thanks. And Good Luck!