On older games ( 80's, like Viking ) I have seen many times where the machine was not stored properly and thus the Left & Right sides of the backbox have started to bend downwards towards the floor, making the bottom channel sort of banana shaped & not allowing the backglass to sit correctly in the channel, thus not being able to lock it in place.....

...Never heard of a Repro glass for Viking....Who produced it & when?

There is a huge difference between a "Reproduction (or Repro)" and a "Restored Original".......