hi firemaiden ... Despite the slight difference, the symbol on the card is likely the Aum symbol:
(1) Aum is one of the best known yoga mantras, and MP does refer to yoga in her text.
(2) Being a calligraphic symbol, traditionally there are slight differences whenever the symbol Aum appears; any such difference can be explained this way too. MP could have taken slight artistic licence too.
(2) It's interesting too that the Aum symbol looks like the number 3, corresponding to the "3" of Coins.
(3) It's also interesting that in the article http://www.reikihealinglight.com/omaum.shtml,
they speak about the symbol Aum representing 3 states; thus, an another linkage to the number 3. However, I'm not sure if this linkage was in MP's mind. MP talks about 3 planes, albeit different from the 3 states referred to in the reikihealinglight article. Still, we have the number 3 creeping up again.