I'd have to see a contemporary comparison for the screws. Bot screws in the trigger guard are brass with wide wood threads
The original screws are iron, not brass, and once in place, the heads were polished down even with the guard - the two screws in the butt plate should be the same way. These screws were hand made and hand fitted, and they don't really interchange well except by accident.
I guess I mentioned this only to suggest these will make a difference to a serious buyer. For me, each of the four screws, if original, and unturned, adds $60 or more EACH to the value of a Birmingham Model 1853 compared to flat topped brass screws (from a door hinge, I would think).
I guess it's like judging beauty of a person - a favorite historical character is Sir Harry Paget Flashman (brigadier-general, V.C. [Victoria Cross], K.C.B. [Knight Commander of the Bath], K.C.I.E. [Knight Commander, Order of the Indian Empire]: Chevalier, Legion of Honour; Order of Maria Theresa, Austria; Order of the Elephant, Denmark (temporary); U.S. Medal of Honor; San Serafino Order of Purity and Truth, 4th class), and when judging what he calls a ladies'
"upper works" which have been artificially augmented, one judge might think this an improvement and rate higher, but another might rate a change for original condition lower!
I really like the Flashman series of books. He is the only character who uses this phrase "upper works" to describe a lady's form.