There actually are four different options:
ETS16005: 1 regular gun
ETS16006: 1 tank version (as used on the FT-17 (FT-31)
ETS16007: 2 regular guns (economy option)
ETS16008: 2 tank version (economy option)

There actually are four different options:
ETS16005: 1 regular gun
ETS16006: 1 tank version (as used on the FT-17 (FT-31)
ETS16007: 2 regular guns (economy option)
ETS16008: 2 tank version (economy option)