My answer was not specifically related to the B737, but rather to a general idea about how a strut works.
I'm not familiar with all the details of B737 systems since I don't have a typerating for that type.
(I fly AVRO RJ85 and RJ100).
I have however a DC-10 manual here at hand which I used to describe how a hard landing is dealed with.
I've looked it up once more and the part I've described is called a 'crush tube'. You can find it at the top of the nitrogen chamber.
Just to make clear to others how the system works:
The stut (= leg of the landing gear) is made out of 2 concentric cylinders. The upper cylinder is attached to the fuselage, the lower cylinder to the gear.
When the landing gear makes ground contact, the lower cylinder is pushed into the upper cylinder which is somewhat wider. Both are filled with oil and the upper cylinder also contains some nitrogen gas. Because of the reduction in volume inside the lower cylinder, part of the oil from the lower cylinder must go through one or more small holes to reach the upper cylinder, thus loosing energy. Also, the way through is progessively blocked by a cone-shaped metering pin attached to the lower cylinder and moving upwards as well. This makes it more and more difficult to compress the strut even further. The nitrogen gas in the upper cylinder acts as a spring which is compressed and will slowly expand again to its normal volume, pushing the oil back into its chambers in the lower strut. However this is done much more slowly, so only about 10% of the impact energy is released again. The rest is transformed into other forms of energy like heath.
If the nitrogen gas is compressed to its mimimum volume and this still isn't enough to absorb all impact energy, then a diaphragm on top of the nitrogen gas chamber folds over and a crush tube is used to absorb the rest of the energy and to act as a rebound control.