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Should the 51pf capacitor in the deadringer be a ceramic or polyester cap?

A metal box is already shielded, especially an aluminium box. No need to do much else to it. If the circuit inside picks up noise it's most likely coming from the power supply or the guitar.

With most input/output jacks I've seen used in diy pedals, whichever method you use will end up with the metal enclosure being "grounded" when there's a cable connected to the input jack. Effectively the battery negative pole, supply negative pole, instrument cable sleeves, circuit ground and metal enclosure being at the same electrical potential - same point in the circuit. If you test for shorts between any of these points you'll see they are in fact shorted. Sorry for my lousy english.

As for the need for metal enclosures. You need some kind of shielding especially with the dirtbox circuits. This doesn't necesserally mean you need a metal enclosure, it could be a wooden or plastic box, but shielded inside (or outside) with a foil of some conductive material. A metal enclosure or shielding foil in a non-metal enclosure acts as a Faraday cage. This stops electromagnetic interference from getting inside the box and being picked up and amplified by a high gain circuit. I've had distortion circuits pick up my heartbeat, even radio transmissions from halfay around the world when testing them outside an enclosure, but they've worked without any noise inside the enclosure.

As said above, for the two examples you've given, there's no difference for pedal builds. Though for some opamps there might be variations that can affect the sound, so it's best to check the datasheet if you're not certain.

Hi

I've built the Neutrino black version with a PCB I etched myself. But I can't get any sound from the output. Should there be a short in place of the omitted diode (D4)? Also is C2 just left out (nothing in place of it)?