Regarding the Raleigh seatstay caps...
I can't be sure without seeing a picture of the Raliegh caps, but based on your description, my '74 Jack Taylor has the same type of caps. Since they only made 200 to 400 frames per year, I doubt it was done to speed up production.
I vote for Mark Stonich's theory: "you wouldn't count on a nice little mini-fillet forming on the inside. But you could be sure of one on the outside."
Duncan "eagerly waiting for Tom Kellog to finish re-painting my '74 Jack Taylor" Granger Lancaster, PA