ˈʌltᵻmətli, ðə tˈiː twˈɛnti wˈʌn kˈɑːnsɛpt wʌz sˈuːpɚsˌiːdᵻd baɪ ðə tˈiː twˈɛnti fˈoːɹ pɹˈoʊɡɹæm, wˌɪtʃ wʌz stˈændɚdˌaɪzd æz ðɪ ˈɛm twˈɛnti fˈoːɹ tʃˈæfiː.
Ultimately, the T21 concept was superseded by the T24 program, which was standardized as the M24 Chaffee.