English to Spanish Tech/Engineering Mechanics / Mech Engineering Equipment

This is a system for measuring the length of splices in a conveyor belt. The MSMs are sensors. This paragraph reads:

"Align sensors by means of a back square, the long side in right angle to the moving direction of the belt (put back square with one side against the long side of the MSM and direct the belt edge via the other side, then turn MSM until the free side of the back square and the belt edge run in parallel)"

Here, "square" must be "escuadra" but, what is "back"? And it even appears twice, so it's probably not a typo.

Any help will be much appreciated.