Learn Swedish: Jag kan se vilken schablon som inte har signaturen. - I can see which stencil does not have the signature.

This is a crucial rule for indirect questions in Swedish! When the question word or phrase (here, vilken schablon) is the subject of the embedded clause, you are required to insert som. Even though it feels like saying "I can see which stencil that does not have..." in English, this som is completely mandatory in Swedish.

Would I still use som if the sentence was "I can see which stencil you are using"?

No, you wouldn't. If the question phrase is the object of the embedded clause (because "you" is doing the action), you omit som: Jag kan se vilken schablon du använder. You only insert som when the interrogative phrase itself acts as the subject.

Why does inte come before the verb har here?

This is a great reminder of the "BIFF" rule! An indirect question functions as a subordinate clause. In Swedish subordinate clauses, sentence adverbs like inte must always be placed before the finite verb. Therefore, it has to be som inte har, never som har inte.

Is schablon a common word, and how else might I see it used?