Here are some of the serial numbers i have found, they give an example of the pattern i am seeing (Engine, Years Run, Serial):
S70 MK1.0 (1970-1971) - 5270010321
S70 MK1.0 (1970-1971) - 5270033448
S70 MK1.0 (1970-1971) - 527001866
S70 MK2.0 (1971-1973) - 5271051074
S70 MK2.0 (1971-1973) - 5271014993
S70 MK2.0 (1971-1973) - 527202648
S70 MK3.0 (1973-1974) - 44730954
S70 MK3.0 (1973-1974) - 797401401
S70 MK3.0 (1973-1974) - 11074056700
S70 MK4.1 (1974-1977) - 1107508081
S70 MK4.1 (1974-1977) - 1087601265
The 2 digits that start with 7, either positions 3&4 or 4&5 always seem to fall within the years the engine was run.