I'm not sure why you suspect it was once a flintlock?
One of the first things to look at when the gun is suspected of having once been a flintlock is the stock wood, right under and behind the hammer.
Notice that on this gun, the wood totally covers the top edge (face) of the lockplate and all around it.
This tells me that the stock was never cut for a flintlocks use.
Most flintlocks have the wood under and behind the cock removed so the top surface of the lockplate is exposed in this area.
The inside surface (towards the barrel) of the cock above the lockplate usually projects inward so that it hangs over the lockplates upper surface.
The projection, which extends upward towards the flint jaws is removed in the area where the face of the lockplate exists. This stop serves as a "stop" for the falling cock.
Because the stop needs access to the top of the lockplate to make the cock stop falling when the gun fires, the wood in that area needs to be removed.
The length of the removed wood extends towards the rear of the lock so that it clears the cock at all times, from when the cock is fully cocked to when the cock rests in the fired position.
Yes, there were some flintlocks made with internal stops for the falling cock but most used the exposed top of the lockplate method.