I can't speak for the Pietta units as I only have a Uberti, but the flash channel is smaller than the factory one.
These Uberti are 12X28 & have flats, not a hex. The Uncle Mike's spec lists them as " Uncle Mikes #2866. 12X28tpi, Same as for: Ruger Old Army. Thompson Seneca rifle, Colt revolvers" What cat # are they listing for the hex head versioins? Maybe there are more than one 12X28 variants?
IIRC the Uberti has a longer cone than the Pietta & a different thread as well.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”