The Options Clearing Corporation puts out an information memo for each adjustment that is made that explains the adjustment. Infomemo 31854 was issued for Abbott on Dec 12. It states that effective Jan 2, 2013 the option symbol changes from ABT to ABT1, no change to strike prices, no change to the number of contracts, the multiplier remains 100, the new deliverable per contract is 100 ABT common shares and 100 ABBV common shares, the settlement allocation is ABT: 50% and ABBV: 50%.