IBM AC Adapter (16v/ 4.5A) - 08K8204. This model works with any of the listed laptops below:

Compatible Models
IBM Thinkpad A20
IBM Thinkpad A20E
IBM Thinkpad A20M
IBM Thinkpad A20P
IBM Thinkpad A21
IBM Thinkpad A21E
IBM Thinkpad A21M
IBM Thinkpad A21P
IBM Thinkpad A22E
IBM Thinkpad A22M
IBM Thinkpad A22P
IBM Thinkpad A30E
IBM Thinkpad A30M
IBM Thinkpad A30P
IBM Thinkpad A31E
IBM Thinkpad R30
IBM Thinkpad A31M
IBM Thinkpad A31P
IBM Thinkpad R31
IBM Thinkpad R32
IBM Thinkpad R40
IBM Thinkpad R40e
IBM Thinkpad R50
IBM Thinkpad R50e
IBM Thinkpad R50P
IBM Thinkpad R51
IBM Thinkpad X20
IBM Thinkpad T21
IBM Thinkpad T22
IBM Thinkpad T23
IBM Thinkpad T30
IBM Thinkpad T40
IBM Thinkpad T40P
IBM Thinkpad T41
IBM Thinkpad T41P
IBM Thinkpad T42
IBM Thinkpad T42P
IBM Thinkpad T43
IBM Thinkpad T43P
IBM Thinkpad X21
IBM Thinkpad X22
IBM Thinkpad X23
IBM Thinkpad X24
IBM Thinkpad X2660
IBM Thinkpad X2661
IBM Thinkpad X2662
IBM Thinkpad T20
IBM Thinkpad X30
IBM Thinkpad X31
IBM Thinkpad X40