Volume cap was added to the release of the ZX707 and A306 for US, Canada, Singapore, in addition to EU.... and ZX707 is volume-handicapped From what I've read, they added volume cap in US version (maybe global as well?). They used to do it only in EU versions, but now expanding into other territories...
https://thewalkmanblog.blogspot.com...x700-and-nw-a300-walkmans-not-only-in-eu.html
The WM1A/Zm2 was released with cap only for EU models.
The following release of the ZX707 and A306 had the extra volume caps applied.
Speculation is that the new countries have imminent hearing protection regulations in the works.....