The card I’m looking at (an LFRX variant) has the proper checksum in
0x1F (0x17), but then byte 0x20 has 0x04 with the remaining data 0xFF
I previously thought I understood 0x20 began the region where we could
do what we wanted, but I’m now not so sure. If I was going to store some
custom data, where does that region begin and what is the significance
of the 0x04 in 0x20?