[asveikau]

Wikipedia for Old Spanish says there were two distinct variants of z:

    The affricates /t͡s̻/ and /d͡z̻/ were simplified to laminodental fricatives /s̻/ and /z̻/, which remained distinct from the apicoalveolar sounds /s̺/ and /z̺/ (a distinction also present in Basque).

https://en.m.wikipedia.org/wiki/Old_Spanish#Sibilants

That's pretty interesting. I don't have any experience distinguishing between sounds like that but reading around I start to wonder if I might produce such differences without being aware of it.