The lancers do not have a "chain plate" in the classic sense. It is a flat plate to which is welded a vertical plate with the hole in it for the turnbuckle clevis pin. The "plate" pulls up against the side deck. There is no other support.
I know I pulled mine out on my 1976 Lancer 28 and re bedded them, but for the life of me I can't remember how I got that bit of vinyl liner down fronm the inside of the side decks. Check for hidden screws. Or just pull down and see if there is a point where you can see something holding it back which you can then release. I know I got mine back in place after the job so it couldn't have been all that tough. The plates themselves pulled up against a rough, badly laminated surface. The fit was awful. Always surprised me that they worked at all.