The only time I can see Simon's hand is in the competition pieces of 1651. Even 'fine work' pieces dated 1651 are of a much lower standard and probably, as you point out, cut by the under-gravers. Upon re-reading Oman it is now clear to me that he is referring to reproducing the breeches design for the 1651 milled pieces. Simon probably, therefore, had little or no involvement with the original 1649 engraving, maybe assisting only in helping to sketch out a design (?). Brooke then is also correct, in referring to the 1649 pieces, by stating that they were indeed the work of the under-gravers.
Clear now, thanks👍