Odd repair, maybe a "window" cut into the apron to repair the actual rail. Take a look at the left rail & apron, and around the cowl/firewall/apron area for signs of distortion or repair. The buckle above the rear K mount looks suspicious to me too, as does the lack of undercoating, but I don't know '71-up B bodies that well. Welding the K to the rail indicates the mounting holes may be damaged, or the K was used to pull the rail into position and the shop neglected to finish up, but that's not the proper way to pull & square. Time to check & measure the structure. Are you rebuilding the suspension because of weird handling, or because it's worn?