Cut stock question
 

What percentage would you devalue a Parker shotgun, or any fine side by side shotgun, for that matter, if the stock has been senselessly cut and the original buttplate has been replaced with a recoil pad?

The percentage of value may be as little as 10 or 15 percent, but the available customer base will take an even bigger hit.

The biggest question is what the LOP is with the pad on it? If it is at least 14”, that helps to minimize the reduction in value. As most people can shoot that. Any shorter and it narrows the market.
The quality of the work and install of the pad also come into play too.

I'm surprised that the hit would be only 10 to 15%. I would have guessed a third, minimum. It seems reasonable that interest would significantly diminish. In fact, I'm only interested if it can be had at a very low price.

Grade, gauge, condition and other factors could also play a role. When I bought my GH the stock had been cut way back, poorly repaired, painted over by hand and a redhead pad installed. The LOP was still only a little over 13” so a new stock was definitely in the cards. The gun was 16ga, 0 frame with uncut Damascus barrels and I liked it otherwise so despite not having a Manson gauge I took a chance and offered the dealer ~15% of what I felt the gun would have been worth with a decent stock and safe barrels. The barrels were safe, Brian restocked it and the gun turned out beautifully.