I think if the barrel was swapped aftermarket and you have the original (important-undamaged from the swap) barrel, it’s value is still there plus the considerable value of that barrel. The proof mark it has on the barrel doesn’t help as every true accessory pistol barrel I have seen is individually proof marked.
Lots of “ifs”. If the original barrel is lost, it’s value is lowered as well. It’s still worth maybe shooter grade money (which isn’t exactly cheap with M13s, they are all collectible to a point lol) plus the barrel of course. It wouldn’t surprise me if it brought more money than a comparable shooter solely because of the added worth of that barrel. It would raise the price of anything it was on, including if it was just sitting on a box not attached to any P7! However, if the host M13 cannot be returned to “as it left factory” it’s value is lower than if it could be. So, I would think in order of value:
1. Factory P7M13SD
2. Factory P7M13 with both its original non threaded and a true HK accessory threaded barrel
3. Missing the original barrel but has this nice threaded one.
I think this might bring more than even a matching but shooter grade one, but that depends on the buyer, and really has everything to do with the barrels collectibility by itself, more so than whatever mix matched M13 happens to be on. It would have been worth (I believe considerably) more if #1 and 2 above.
Without verification I would think you have to approach this as though it a #3 in my examples. Respecting forum rules and your original question I won’t comment on “what (I think) it’s worth”, only in a limited way where I think it would sit in the market compared to factory options.
I really think if you dig as suggested above you might be able to find serial number ranges that would be helpful. The gun is likely worth the investigation if reasonably priced. 👍
Even if it is not original....if you are having “Suppressor Range Day” with your friends and pull this out...I think you are likely to win the cool pistol category IMO lol