 The early Y @-@ 1222 model Fat Man was assembled with some 1 @,@ 500 bolts . This was superseded by the Y @-@ 1291 design in December 1944 . This redesign work was substantial , and only the Y @-@ 1222 tail design was retained . Later versions included the Y @-@ 1560 , which had 72 detonators ; the Y @-@ 1561 , which had 32 ; and the Y @-@ 1562 , which had 132 . There were also the Y @-@ 1563 and Y @-@ 1564 , which were practice bombs with no detonators at all . The final wartime Y @-@ 1561 design was assembled with just 90 bolts . 
