Long ago...and far away....you could buy the metal attachments you are talking about as they were offered for sale by the company that made the Believer at that time.

If my memory is accurate....it may have been that the at least some Believers came with the metal attachment (which was basically a Cisco kid style lip) that fit over the line tie(s) and had a pin that was put through to hold it in place ...sort of. The pin did not go through the lure body.

 On a related note-I have been fiddling around with trying to alter 10" Believer action using#10 Colorado blades ,curving up,  with 2 slots cut to go over the line ties. The blade is attached using a  Cowling Lock Pin ..(see google images)....the snap from the leader does the same thing on the second line tie. So far I have tried only casting with this set up using #9 Indiana blades as well.  The first #10's overpowered the lure so I went to the 9's but will try the 10's again. When casting, I have only been using the rig as a jerkbait and it gives interesting action, especially since  the slots for the blades have enough clearance in them that the blade is loose and  able to move some from water pressure and you get some unusual moves from the lure. 

  I hope the fish agree this year.  Trolling so far has been only successful at very slow speeds so it is rather a self-defeating deal there.....but I still want to keep experimenting.

For everything,  only .025 , thin blades were used as heavier ones seemed to kill the   action.     Hope this might be a starting point for some  experimenting on your part. Good luck.  MacGyver is alive and well.