My Bosch parts list shows:
P/N
1 464 619 320 = 36.90mm long spring
1 464 619 318 = 37.10mm
1 464 619 314 = 37.40
This indicates a length variation is used to change the advancement curve. Unless they also have different spring constants.
I also noticed that there are no other shim sets offered, maybe indicating you swap them around to get different response curves.
Anyone see different spring numbers in their Bosch parts list for their I/P?