Can a weak entity type be involved in an "is-a" relationship?
When I've modeled databases, I would use a isType description where you have isA.  You appear to have two types of Specs; labelSpecs and itemSpecs.  Common data belongs in Specs entity, with type specific data in the appropriate subtype record.  Normally subtypes are exclusive, so a Spec would be either a labelSpec or an itemSpec.