Not sure what vintage 520 you have but here's a couple of links:
I've bought Anschutz mags from this fellow - Currently showing out of stock but worth an email as he may be able to put you onto a clone.
https://riflemags.co.uk/anschutz-525...gazine-525-u8/
Nordic Marksman in Canada used to sell them but are also showing them as "out of stock". I've bought a lot of Anschutz spares from them - Always very helpful (as long as I have the correct Australian import permits!!!!)
Good luck finding one as it won't be easy!
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