There is still a bit or research being done on these, special procurement / project is thought to be the meaning of the SP.
Are they an additional contract? Unlikely as they have not marked others in the same way, are they a contract outside the MOD for another customer? (My hunch), or are they a testing / prototype batch. The jury is still out.
They do tend differ from the typical format, this watch has nice patina, an SBS dial in stainless case.