Tom, Digger,
Like Digger said, there were two different fuel caps used, P/N 61 635 C2 for S/N 506209 and below, and P/N 109 037 C1 for S/N 506210 and above. The TC-157 parts manual I have (Rev 24, dated 2/81) list only one number for a fuel tank, P/N 63 719 C91, and says it replaces P/N 61 927 C2 by also ordering the cap for the higher S/N tractors, so it appears the earlier tank was discontinued at some point.
Like Digger said, there were two different fuel caps used, P/N 61 635 C2 for S/N 506209 and below, and P/N 109 037 C1 for S/N 506210 and above. The TC-157 parts manual I have (Rev 24, dated 2/81) list only one number for a fuel tank, P/N 63 719 C91, and says it replaces P/N 61 927 C2 by also ordering the cap for the higher S/N tractors, so it appears the earlier tank was discontinued at some point.