This base has a hallowed recess so i can put a all-ready based model into it. For most bases you will need to trim off the side lips and pegs, then sand down the existing base to sit flat in the recess of this base. So sadly it's not a drop in solution, and you will need to do some minor rebasing work. But for rebasing 40mm squares into 50mm rounds this was the best I could figure out.

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