Note that where the spring contacts the alloy there is a little channel routed out. Did this to ensure the spring moved freely without binding. Also note that in the normal derailleur, only the top of the round portion rides against the underside of the outer plate; in my hollowed-out version, the load is spread evenly on the two "rails". While I simply wouldn't trust it under really heavy power shifts or for a long-term option, it does still function the same way as before the drilling.

As for the failed one, well, it was just subjected to a load unlike what it would experience during use while being assembled. In any case, lesson learned. I only did two more (with simple drillings on the back plate) with a hollow front plate and sold them on Ebay a couple/three years ago. I don't think I'll be doing any more unless someone commissions one. I did that for Poprad, for example.

DD