Aluminum washboard kit

[redguy]

Just received my new washboard kit for my GII and can't wait to try it out.
The only thing that is not obvious (no instructions) is the purpose of 2 little squares of rubber/neoprene, about 2" square.
If anyone can tell me what they are for I would be very grateful
Everything else looks pretty straight-forward, and I see that they should stiffen things up considerably