Set Value to Parameter within .rfa

Hello all,
This topic has popped up once or twice on this forum but to my knowledge there have been no full complete solution (I could be wrong). Simple, I want to populate a value to a parameter for multiple .rfa files without opening them. I have done this script which is running error free but when I open the family after , the parameter is not populated at all.

Any ideas where Im going wrong would be much appreciated.

10_Set Parameter value to Family.dyn (25.1 KB)