This particular W3 was decomisioned from battle status, after it was hit in its arm, and had to be put in for repairs. Unfortunately it was never deamed combat ready again, and instead became a construction robot, the W3-C. The differences between this unit and the standard W3 are in its arm of course (taken from a W1), and its lack of headlights.
Brilliant compact design and I love the history you've created for it, very imaginative. Yellow is such an underrated colour - I have a selection of construction mechs on my own MOCpage but I went bigger with my designs...this little guy is a real work of ingenuity! ;)