Trying to understand basic packet forwarder on Kerlink Gateway

That is true:
A1 = 0x01 | 0x00 0x00 0x00 0x00 | 0x00 | 0x03 0x01 0x01 0x02 | 0x5B 0x00 0x00 0x00 | 0x00 | 0x01
S1 = 0xe6 | (15 more bytes)
Payload = 0xA7 (0b10100111)

S1 XOR Payload = 0x41 => 'A' in ascii