How Can I get the opposite of this relative transform


I have an object that’s attached to the hand bone and positioned to fit the hand pose and I have the relative transform for that.
Cool!

I have some weapon attachments on weapons & I want to grab them while they are on the weapon so I Now want to do it the other way around & Attach the hand to the Attachments and somehow use the same transforms each one has to keep the same hand position.

things have not been going well here.