I have the NT-510 and love it and I believe the NT-520 just has a different style of buttons.

I'm not 100% sure if I'm remembering correctly but the NT-530 was different enough that someone on here found it didn't do everything the previous models did.(as said before not sure)

Try Amazon as the sellers there sometimes have a 510 for $99 USD

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"When the Team Chief said.... You're trapped in a hole with nothing but a goat and a slinky, what do you do? Stubby said, I'm not sure but it won't end well for the goat...." ~(Overheard) Last day, Phase 3, Q Course