I wonder how wide the bottom margin is. It appears very wide and shows no trace of an adjoining stamp. That would suggest a bottom-row stam, hence TA.
On the bottom left, measuring as is from the end of the red to the bottom of the corners is about 1.5mm.
Your thinking is along of what I had tried to figure out the other day. I have a bunch of Penny books, etc., and I know I've seen measurements. But for the life of me I couldn't find the measurements and gave up before 'googling'. I also had wanted to make a static template to place scans into to get a visual of missing edges. But gave up!
Something as simple as scanning to actual size I couldn't figure out and got frustrated.