Found on the internet:
Super 55
 1954 6 001 – 8 290
 1955 11 837 – 31 370
 1956 35 001 – 43647
 1957 43 916 – 56 036
 1958 56 501 – 59 033

Also found that on other models, one of the casting numbers indicates the date the block was poured. 
Would the casting number S121454 indicate the block was poured Dec 14, 1954? And therefore my tractor is the &quot;early version&quot; since  SN 15981 is considered the &quot;late&quot; version (mid 1955)?
Help me out here, please. Thanks.