As you suggested, Northwest guns generally had brass buttplates, with brass "serpent" sideplates and ribbed brass ramrod pipes, but an iron triggerguard. However, if you are determined to have a steel buttplate, you could probably make one if you have some basic workshop tools (vise, hacksaw, files, grinder, and drill). Based on the work you have already done with your trade gun, it sounds as if you are pretty handy with tools, and I'll bet you could do a good job.
The buttplates on Northwest guns were typically made of flat stock, with the body of the buttplate left flat and the return simply bent to a 90 degree angle, although the actual bend was slightly radiused and not a sharp corner. The buttplate on this Northwest gun has quite a lot of radius at the bend, although certain features of this gun make me think it was re-stocked at some point: The moulding around the lock looks wrong, and the "teardrops" are very crudely carved. The wrist is too thick and there is no "baluster." However, the stocker probably recycled all of the original hardware.
View attachment 29937
The
Trade Gun Sketchbook has buttplate patterns for all of the guns within it, but you could probably just trace your original brass buttplate to make a paper pattern. The thickness of the plates on original guns varied from 1/16" to 3/32" to 1/8", according to the aforementioned book. Transfer the pattern to a piece of mild steel of the appropriate thickness and use the tools you have to cut it to shape and bore screw or nail holes. If it were me, I would fit the buttplate to the wood, rather than the wood to the buttplate, especially if you have already refinished the stock. This would allow replacing the brass buttplate if you decide you want to put it back on.
You always have to factor in your time on projects like this, but I think spare parts for Pedersoli guns are pretty expensive, and it sounds as if the steel buttplate you want is not available at any price. Maybe you could go to the Classified section of this forum and offer to swap buttplates with someone who has one of the steel ones on his trade gun. Just a thought...
Good luck with your project! Let us know how it turns out.
Notchy Bob