What kind of 1900? A ProV? I would guess it is secured to the floor, perhaps under the carpet, by screws. It could have some sort of L bracket and rivets though. It probably has the same/ similar fasteners running up the port gunnel as well, so it is secured on both sides. You might have to peel up carpet and drill out rivets or something.
IDK which dealer you called, but R&R marine in Shakopee, MN has always been super helpful with anything I need. Or try calling lund directly?