It is part of the DEWALT TSTAK system, which allows you to stack multiple boxes together for easy storage and transport. Compatible with original TStak cases. HS 1-9, I 1-56, KA27-28, KW1-17, PA20-49, PA62-77, PA60-61, PA78, PH17-26, PH-30-44, PH49-50, ZE1-3, BT1-94, IM1-9, IM99, TR21-25, Isle Of Wight.
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