Your tractor was built in October 1978. The last 6 numbers on your S/N, 638629, determine this. The rest denotes the model of tractor, which is indeed a 1250. The "U" means built in the US.
Hope this helps
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'70 107 with k301 engine swap
'71 106 with 38" deck
'70 147
R with factory replacement k321, 42" deck
'61 Original with 38" timed deck
'63 70 "pinkie"
1863 with 54" deck
'46 Farmall H, '50 Farmall Cub
105 x2 (parts)