So for example, using your FpNr of 01797C - letter stamped 23.6.40 the entry for that is Stab I, 1.-4. Kompanie Infanterie-Regiment 40. So the C at the end of the FpNr indicates 2 Kompanie as A would be for Stab, B for 1 Kompanie and so on.
Thank you Hucks - using that guide I will do some more translating of the letters and see what I come up with.
I still think it will be problematic unless I have the letters home from more than one member of the family?
Thanks again for your help.
" When you're chewing on life's gristle, don't grumble, give a whistle "