56 Ma
I eyeballed the card and made an initial assessment of 60/40 once I had card in hand. It is in the case and I don't have a micrometer.
However, upon the questions, I brought the ruler out that is marked off and labeled as 50. This is an engineer ruler for scale drawing purposes. 50 ticks on the ruler = 1 inch. This would mean the resolution of the measurement is pretty good and under heavy light and magnification should have an accuracy as best I can of +- 2%.
I measured in the picture as noted (middle of the card). The left side (or top of the horizontal presentation of the card) yields to the best of my ability 6/9.5 and the right (bottom of horizontal) is 3.5/9.5 which is 63/37. So, I was off on my eye ball measurement and given the tolerance of the ruler and "best as I can with these old eyes" it appears to fall between ~62/38 to 64/36, so not quite 60/40 of my initial assessment but much better than the 75/25 guess.
Regardless, I'm extremely happy with it even if the T/B is 63/37 and not the previously stated 60/40.
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