Modified frame number

Hi all
Need a little help as to how this "modified frame number" is generated. 
I am comparing two systems each recording the same ARFCN at the same 
time while I
am connected and making a call on my own phone. System A is USRP1 and 
Air probe while system B is Motorola C123. with Osmocom ccch_scan.
  With the Air probe capture I have all needed for XOR,Kraken and 
Find_kc. However with system B I have same info but I am missing the 
"modified frame number". Even with this I think I have enough to run XOR 
and Kraken but will not be able to match the key using find_kc.
Can someone explain how this number is generated.

Tom