Would love to get some specs on these diodes if possible. Maybe they go by a different device number?

I picked these diodes up at a vintage Audio get together sometime back. The only thing that is marked on them is NDP 590. I saw someone at one point selling NDP 420 diodes, but I can’t find any info on those either. I have a few hundred of these and would love to use them. I believe I have found a use, but would really like to get a voltage and current rating. Can anyone please help me?