I understand what you are talking about Ed - new locks and breeches to fit the original tang would not be a difficult change, however.
I see the plugged screw hole you are talking about, but think these are cap lock plates from the get-go. I am not familiar with such rear screw hole, behind the pan location. I think rather, this are lock plates from another gun, made to fit this one, by filling the cross bolt and re-drilling and tapping a hole further back, above the bolster boss.
That hole up front does look like a feather spring hole, however, might simply be the hole for the main spring pin.