Within Delta there are 8 different configurations of 757-200. They can either be broken down by their designation, i.e. 757, 75A, 75E, 75J, 75M, 75N, 75V and 75X. Or as in the case of the pre merger Northwest planes they are also offen refered to by their tail number as "5500 series" or "5600 series", basically the 5500s have tail numbers 5501 thru 5534 (though many are missing having been retired or stored) and the 5600s have tail numbers from 5635 through 5657. The 5500 series are the older Northwest planes with the smaller door exit behind the wing and the 5600 series have exit rows over the wings.
Here's a basic run down of the Delta 757-200s, though I may be off on the number of each type in service:
757 is the 24/156 configuration. Aircraft are in the 600 number range, 51 in the fleet.
75A is the 24/150 configuration (was the 16/144 NW
trans atlantic config). Aircraft in the 5600 range, 8 in the fleet.
75E is the 16/158 configuration with BizElite and AVOD/PTVs. Aircraft in the 6800 range, 17 in the fleet.
75J is the 20/159 configuration used for inter-Asia flights. Aircraft in the 5600 range, 8 in the fleet.
75M is the 22/159 configuration. Aircraft in the 5600 range, 7 in the fleet.
75N is the 22/160 configuration. Aircraft in the 5500 range, 14 in the fleet.
75V is the 22/153 configuration used for Hawaii flights. Aircraft in the 6900 range, 4 in the fleet.
75X is the 26/158 configuration with LiveTV and AVOD/PTVs. Aircraft in the 600 and 6700 ranges, 48 in the fleet.