300E Expansion tank mounting

I went to change my expansion tank on my 1987 300E, there are the two 10mm nuts to remove from the studs in the fender well.
There is also part of the tank that presses into the inner fender. I really
had to pull hard to get it out. There was washer that is sandwiched together made out of rubber and plastic that attaches to the part of the tank that presses into the fender.
When reinstalling this does the plastic / rubber washer go on the tank then press into the fender well or do you press the tank into the fender well and then press the rubber / plastic washer onto the tank from the underside, the wheel well area?

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