@FSU, you're correct... it's the central clearing... But I'm pretty sure the pro-rata methodology is used. @Robert Morse? You're broker deciding who gets what... that's just fxxcked up! It should be all pro-rata...
It looks like there are two approved assignment procedures. I'm not an expert on this. PRO RATA or Standard-Assignment (wheel). https://www.theocc.com/components/docs/legal/rules_and_bylaws/prorata-assignment-procedure.pdf https://www.theocc.com/components/docs/legal/rules_and_bylaws/standard-assignment-procedures.pdf