Bearing in mind that my car is a DHC, the width of the “A” post seal retainer on mine is 11/16" while the depth is 5/16", however, not sure if this would be the same as on your OTS?
Nick, I have a late '50 OTS and the inside width of mine as measured in several different places – both sides – comes in at 0.625" (5/8"). Also, bear in mind that each side doesn’t have a single top-to-bottom run. For my year car they are actually two different pieces for each side. That is to say, the upper piece comes down to the level of the top of the sill. From there, a separate steel (as opposed to the upper aluminum) piece of the same dimensions in-line with the top piece continues down to the bottom of the sill. I believe mine are original as they had traces of the original paint under four subsequent re-paintings.
Same 5/8" on my Sept '50 OTS on both sides. Don’t know if originally done at factory but on the top of seal retainer the metal on both sides has been hammered or slightly pushed down to finish the edge (on the outside edge only)