I think these heavy cord (major through general) boards date from the 1866 pattern. At that time, all field grade officers (Ltn. to Hauptmann) wore the flat, narrow 1866 pattern. When the next major change came in 1888, I believe the higher ranks cords were downsized to more what we would expect to see, say, in 1910. Officer's still had some leeway in their choices and I'm sure many older officers continued to wear the larger boards after 1888.
Chip