There are two different WS-C5110 form-factors released over time.
The first is a Dell R1950, model name WS-C5110-SR.
This type started shipping on April 10 2009, and has a hardware rev (at end of serial# NNNNNNNNNNNX) within the range 5A through 5C.
It has its serial number sticker attached on the side (under the mounting rails) or top.
The second is a Dell R610, model name WS-C5110-2-SR.
This type started shipping on January 1 2010, and has a hardware rev (at end of serial# NNNNNNNNNNNX) within the range 5A through 5N.
It has its serial number sticker attached to a pull-out "EST" slide tab located at the front left, under the front bezel plate.
Functionality is effectively identical, between the two units.
Units cannot be subject to RMA just because they have a different appearance.