I am on a similar boat as you, purchased the 4 3/8 BP which i feel was a waste now because it is too small for my 5.5" pads and was causing them to deform. I went ahead and took my original 3401 BP and cut that foam sticking out all around the edge and now 5.5" pads fit perfectly on it WITH a safety margin where the actual pad sticks out far enough to use safely, and i can apply pressure knowing it is being distributed to most of the pad and not just the center of it.
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