Can't say I've seen a Parker 51 thermometer holder before, but I assumed they exist.
Have to shoot some pics of this (got it late this afternoon). But here is the original eBay listing with decent pics.
Full-sized FP is marked USA on cap and aero filler. Thermometer has no country marking. Cap on thermometer was definitely made to fit the case. Doesn't appear to be a pencil rework. Length of holder with cap on snugly is 4.75 inches.
FP appears to have been inked once. Both pieces very clean.
Not quite on the scale of my Golden Arrow sumgai, but I'm happy.