I always thought that the gun running under the belly looked a lot like the pump from a spray bottle
You're thinking of the Lars family speeder which is parked in the dark. The black speeder 3PO hides behind (it's painted white during the Tunisia shoot).
http://starwars.wikia.com/wiki/Image:Lars_landspeeder.jpg
Here's Luke playing with the model size version of the T-16 (concept model built by Cantwell) with the full size one in the background (behind R2).
http://starwars.wikia.com/wiki/Image:LarsGarage.jpg
The walkway into the house runs right behind the T-16's engine exhaust.
The nose of the family speeder can be see on the far left.
BTW - the original model has been on display a few times so there are great pics of it out there.
Like these:
http://flickr.com/photos/nickstone333/1118018566/sizes/o/
http://flickr.com/photos/nickstone333/1117189787/sizes/o/
http://flickr.com/photos/lrosa/1027023501/sizes/o/
http://www.flickr.com/photos/garrettc/1241338454/sizes/o/
Totally an unexpected surprise to see that, believe me!!!