Complete development history of S-Master describe here:
Walkman
Released in 2009
X Series NW-X1000 (Master)
A series NW-A840 (Master)
Released in 2010
A series NW-A850 (Master)
Released in 2011
Z series NW-Z1000 (MX)
A series NW-A860 (MX)
2012 release
F series NW-F800 (MX)
2013 release
ZX Series NW-ZX1 (HX)
F series NW-F880 (HX)
M series NW-M500 (MX, improved version)
2014 release
A series NW-A10 (HX)
2015 release
ZX Series NW-ZX2 (HX)
A series NW-A20 (HX)
ZX Series NW-ZX100 (HX)
Released in 2016
WM1 Series NW-WM1Z / NW-WM1A (HX・2nd Generation)
A series NW-A30 (HX, 2nd generation)
Released in 2017
ZX series NW-ZX300 (HX, 2nd generation)
A series NW-A40 (HX, 2nd generation)
Released in 2018
ZX Series NW-ZX300G (HX・2nd Generation)
A series NW-A50 (HX, 2nd generation)
Released in 2019
ZX series NW-ZX500 (HX, 2nd generation)
A series NW-A100 (HX, 2nd generation)
https://ja.m.wikipedia.org/wiki/S-Master