It is an air filter. Believe it is the same as JnJ trains http://users.dwx.com/~jnjtrains/products_ frame.htm http://users.dwx.com/~jnjtrains/Aready/0134.jpg
Brad: What you're looking for is made by JnJ as part 344-134 (that's the Walther's code, so it's JnJ's part 134). It's a Horst air filter, in this case (I believe) applied at the Paducah shops when the GP was rebuilt. ICG had a bunch of these, and I know that at least the two US Army units that used to be at Ft Knox also had them. I had fun some years back modelling those units in HO for a customer of a friend of mine who owns a shop down there.