[Mawashi]

I would think the answer would hinge on whether this barrel was added at the factory. If it was not, then it’s value would be lower than one that left the factory so equipped. So you stated that you believe it was fitted a year later. Do you have any ideas about who fitted it?
Either way you have an interesting and valuable gun.

 

[Mawashi]

With no documentation I would assume and treat it as though it was a factory barrel assembled by someone not HK. As stated Gray did some as did others. All would be valuable but not HK factory valuable IMO. If the seller wants more than that would bring, perhaps they should have thought about keeping something that might show provenance. As few P7M13SDs as there are out there, I would think it odd that no documentation/box etc. was kept with one. It seems most knew these were collectible from the start.
These are very niche guns and others on this forum know a good deal more about them but me, including possibly production ranges as well as what if any documentation and items it would have shipped with/is missing.
I also have little to no confidence that calling HK CS or Gray CS would be productive, but I would give both a try, serial number in hand if only to hear them say “we don’t maintain those records”. Lol. The gun could be valuable enough to justify exhausting every avenue you can think of.

 

[Mawashi]

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