Such second bottles are often 3 litres, and may be mounted in various ways from tank bracket to sling mounting.
sˈʌtʃ sˈɛkənd bˈɑːɾəlz ɑːɹ ˈɔfən θɹˈiː lˈiːɾɚz, ænd mˈeɪ biː mˈaʊntᵻd ɪn vˈɛɹiəs wˈeɪz fɹʌm tˈæŋk bɹˈækɪt tə slˈɪŋ mˈaʊntɪŋ.