Denis
I too have a Montrose Doublet, the elastic with the hooks are on the inside of the jacket at the back and are used to secure the Doublet to the kilt, though you may need to have a set attached to the inside back of your kilt to match. I have notfound the need to use mine, the belt goes through the belt loops that go around the doublet. Here is a photo showing what I mean.



Notice the Dress Belt goes around the outside of the doublet.