So as far as per my analysis i found out these, but i still dont know if they are correct. Would be happy and thankful if someone corrected me
$buff = '';
$buf .= pb_field_str(1, $machine_id); // f1: machine_id
$buf .= pb_field_bytes(2, build_os_info(6, 2, $os_ver)); // f2: OSInfo...