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The codex says Zagstruk may be taken instead of the Nob. Does this mean he replaces one of the stromboyz for the additional xx points or that you buy the boyz, then buy Zagstruk as the additional model?
No, the cost listed for Zagstrukk (or Snikkrot or Badrukk) is the total cost of adding him to the unit. The line you're referring to is just stating that you can't take both Zagstrukk and a nob in the same unit (I'm sure Zagstrukk wouldn't want anyone questioning his authority ^_^).
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