Based on what I've seen, I would say the "The MG Car Company Ltd" aluminum has been cut out of the original N-Type sill plate and attached to the replaced sill material.
I've seen several original J & P tubs with the worn sill plates remaining. They were fastened with panel pins - roundish head.
T-Series used #4 oval slotted screws which is a very nicely finished look. Polished stainless sheet metal screws look nice and work well.
Thanks Alan and Tommm. All food for thought . Your post Tommm suggests a new option that may contribute to the confusion. I guess there is no definitive answer! Malcolm