I'm not sure about the u-boot magic number but in the bootloader spec I read the following:
The AIS boot image is expected to be in the user data area of the memory device, written to address 0 of that
region. The bootloader detects the AIS image by checking for the magic word (0x41504954). If the magic
word is not found, the bootloader increments the starting address by 0x200 and tries again. The
bootloader fails if the magic word is not found within the first 2 MB of the memory card.
Typically, the ROM boot loader will first try to detect an SD card. If that fails (i.e., a timeout occurs), the
boot loader will then attempt to find an MMC or eMMC device.
The magic word is at location EA00 on the card which I assume is in the "user data area", falls on a 0x200 address, and is located within the first 2 MB of the memory card.