The one listed as One Size is the adapter plate

Item model number2500604000
Product Dimensions42 x 29 x 26 cm
I cannot for the life of me figure out what purpose this part serves:

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It's located on the rack. There's a little blue notch seen on the right in blue that's spring loaded. If you press it, the entire part slides towards the left, only to revert back to its original location as soon as pressure is no longer applied. When the blue button is pressed, the opening seen on the left is not obstructed. Just a guess, but it looks like it's meant to hold some kind of tool in place.

Does anyone have any idea what purpose it might serve?

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You can purchase an adaptor plate to fit that bike rack from Propel Bikes in California. They cost about $25 each. Ed
JayVee, I know exactly what that part is for. The rack is Carrymore /I-rack made by XLC I believe. When bags made for the I-rack system are inserted into the two cross member with the holes in them, they have two slightly pointed pieces which go into the holes. There are notches in the points which engage in the silver piece in your pictures and they snap in place and hold the bag firmly. Then, when you wish to remove the bag, you have to push on the blue button which then disengages the silver piece from the notches on the pointed inserts and the bag is released. Its actually very quick and neat. The part that I have been trying get for weeks fits this mechanism and is so hard to get, outside of Europe anyway. If you look at the pictures in this article, especially the third one down you will see how it all works.


I would still like to get that darned Adapterplate, also mentioned in my thread.
You can purchase the adapter plate from Propel bikes in California. They cost about $25 plus shipping.