Could it be this:
12X28tpi, Same as for: Ruger Old Army. Thompson Seneca rifle, Colt revolvers?
I had a nipple replacement problem & discovered that "5.5mm x.9" is just the metric description of a 12X28.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”