Weight control was always a major issue with the Lunar Modules. Literally, every ounce was taken into consideration. Grumman saved hundred of pounds from the LM Descent Stage by covering it with multilayer mylar insulation instead of aluminum panels. The Ascent Stage was made of metal thinner than a dime. I've read you could easily poke an ink pen through it.

Are you asking about the black scalloped blast deflector below the RCS quad or something else? The Lunar Module on display in the Air & Space Museum is LM-2 (the 2nd of 12 LM's built). It was intended to fly an unmanned mission similar to the one flown by LM-1 on Apollo 5. However, the LM-1 flight was so successful that the LM-2 mission was never flown. LM-2 has the blast deflectors on it, but like all of the early LMs (1-4), was too heavy to land on the moon. LM-5, the one used on Apollo 11, was the first one light enough to actually make a successful lunar landing. I can imagine the tempation for the Apollo 10 crew was pretty strong, but they would've almost certainly died had they tried to land on the moon.

LM-1 - unmanned test vehicle, flown on Apollo 5
LM-2 - unmanned test vehicle, never flown, on display at the NASM
LM-3 - Apollo 9
LM-4 - Apollo 10
LM-5 - Apollo 11
LM-6 - Apollo 12
LM-7 - Apollo 13
LM-8 - Apollo 14
LM-9 - backup, not flown, on display at the Johnson Spaceflight Center
LM-10 - Apollo 15 - first of the J missions
LM-11 - Apollo 16
LM-12 - Apollo 17

BTW: If you have the proper addin, you can see a panorama of the LM's interior here.

Here's a good site for Lunar Module info.