What an interesting and rare piece! Thank you for sharing it here. From what I can see I do not think there is any reason to doubt its authenticity. However rare, I don't think we need worry that finding it in Croatia is too good to be true. Actually, brass-trimmed armour in the Italian fashion can be seen in Hrvoje's Missal from the early 15th century.
https://en.wikipedia.org/wiki/Hrvoje%27s_Missal
I know quite a few other examples of brass-trimmed armour from this period, but all of them have intelligible inscriptions or simply decorative designs, rather than this kind of pseudo-inscription. I did notice that on one piece from the Met the stippled background is rendered in a very similar way, by filling with small zig-zags.
https://www.metmuseum.org/art/collection/search/35846
Best,
Mark