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The spring tensioner got lost during shipping. Where can I get a limit switch? What is the part number?

They are pretty standard "snap switchees" that can be ordered from many places. If yo're in a hurry, Radio Shack will rob you for one ($3-4).

If it's truly shipping damage, contact SeeMe to make it right.

Here is a link from Ultimachine, the Company who makes the Rambo board.

https://ultimachine.com/content/microswitch-ss-3gl13pt