是的，which they did not know they owned是一个限制性定语从句。
从结构形式上看，这个定语从句是一个主从复合句，只不过关系代词which移位于定语从句句首。不妨把指代先行词something useful的关系代词which还原回去，便可知主从复合句的原形：they did not know they owned something useful，所以，主句为they did not know，宾语从句为they owned something useful，从句是主句的内嵌部分，根据还原的句式再去看待句中的定语从句，指代先行词something useful的关系代词which，是主句they did not know的内嵌宾语从句they owned的宾语，所以，可以把定语从句which they did not know they owned解释为嵌入式定语从句。
个人理解，仅供参考。
