If you want a original looking finish I would suggest using Birchwood Casey cold blue.
This is what I used on this Romy "G" it is not hard to do, and if done right it will hold up just as good as other coatings.
The trick to a good cold blue is preparation, and heat not a lot of heat.
But hot enough that it is uncomfortable to hold for a long time. (don’t burn your self)
1. You need to prep the parts to be blued, sandblast, or glass bead, everything you are going to blue.
2. Clean, with denatured alcohol (if parts are real greasy clean with carb cleaner first)
Trick; when cleaning warm parts put them in a gal. or 2 gal. ziplock bag with alcohol & shake (be sure you are away from any flames or heating source) blow off excess alcohol.
3.Heat, I use a roast pan with enough water to cover parts, get it up to a rolling boil. Put in parts & let it come back to a boil. Grease or oil is a BAD thing at this point if you notice any on parts or on top of the water STOP and go back to the cleaning step.
4.Take parts out of water shake or blow water off, parts should dry quickly. Put warm parts in a 2gal. or larger clean ziplock bag quickly pour in cold blue(I use Birchwood Casey Perma Blue) over parts in bag, zip closed & shake turning over a few times, keep turning and shaking until all parts are covered.
5. Remove parts from bag & rinse in cold water.
6. Reheat parts in the water, at this point when you take the parts out of the water they will turn greenish brown, don't panic! this is normal.
7. Repeat steps 4,5, & 6 until you get the color you want.
8. Heat parts in the water, remove the parts shake or blow water off, put in NEW CLEAN ziplock bag & coat with a good gun oil, I use Remington's Rem oil.
After soaking with gun oil keep parts in the bag & put it in a warm place for 24 hrs. Keeping all parts coated (I put the bag & all on a bunch of shop towels in the back window of the car in the summer sun.)
Follow this procedure & you will come out with a Great Looking Gun like this one.
Only the receiver, front & rear trunnion, trigger guard, and selector was the only parts blued.