(1966–1973): These models have strips of metal welded into the receiver and matching cuts made in the "SE" trigger group to only allow the insertion of an "SE" trigger group.
nˈaɪntiːnhˈʌndɹɪd sˈɪksti sˈɪks nˈaɪntiːnhˈʌndɹɪd sˈɛvənti θɹˈiː: ðiːz mˈɑːdəlz hæv stɹˈɪps ʌv mˈɛɾəl wˈɛldᵻd ˌɪntʊ ðə ɹᵻsˈiːvɚ ænd mˈætʃɪŋ kˈʌts mˈeɪd ɪnðə "sˈiː" tɹˈɪɡɚ ɡɹˈuːp tʊ ˈoʊnli ɐlˈaʊ ðɪ ɪnsˈɜːʃən əvən "sˈiː" tɹˈɪɡɚ ɡɹˈuːp.