That is not a 1N4001. A 1N4001 is a general purpose rectifier, black with a silver band on one side. The diode shown is probably either a 1N4148 or a Zener diode. I also don't know how you can tell how the metal film resistors are precision and low noise, but OK. Anyway, I've been lurking here for about two days and decided to join up. I have my fair share in electronics, I would say.