Ng-model dynamic name , for added material

Ok so here is a code pen CodePen, of a part of my app, I have a button to add more form fields if another unit is there. in my app when you hit the plus it does add another of all form fields. Also the $index variable does display “Unit Information 1” “Unit Information 2” etc.

What I need is to be able to differentiate the information in unit 1 to unit 2, dynamiclly using the $index variable.
I have tried adding the index variable to the ng-model=“unit_loc_ + $index” this does not work. But obviousl does work when just “unit_loc”.

Thank you for any advice in the right way, I am not that fimiliar with AngularJS to much , have learned alot. but I need to make sure I can do this before I go any further with app.

Thanks again