[oletrucks] bedwood milled rabbetts

[varanus@mail.phoenix.net]

Does anyone know why there are rabbetts sawed on the edge boards?  
Both sides of the 2 edge boards are milled and I am wondering why.  I 
thought that the outside might be milled to fit under the stationary 
end rails but I don't know why the inner edge would be milled.

Maybe I am just not thinking hard enough.
Eric Pesci