Seems there are different chipsets in SXT SA and non SA models?
The SA shows 3 chains:
[admin@SNAC002] /interface wireless info> hw-info
interface: wlan1
ranges: 4920-6100/5/a,an20,an40,ac20,ac40,ac80
tx-chains: 0,1,2
rx-chains: 0,1,2
[admin@a67-ac] /interface wireless info> hw-info
interface: wlan1
ranges: 4920-6100/5/a,an20,an40,ac20,ac40,ac80
tx-chains: 0,1
rx-chains: 0,1