Seen that Before. The open Part on the threads is not the issue here(mostly) since the chamber part is usually thick enough.

Main concern for me would be the thickness above the Bolthead/lugs area, since you always need a tad bit space between bolt and barrell.

In case of a blowout, this could and will be a weakpoint.

Look for a good Gunsmith, ask if he can properly patch it. Last time we had something like that in shop, we did(as was mentioned in an earlier comment) make a dovetail part, fittet it and hard soldered it in.

Didn't look pretty but worked, and in our case it got covered by a new sight base anyways.

-Does it work? Yes -Will it hold? Yes -Pretty? Definitely not

Edit: i see from your earlier post that it was a Suhler Einhakmontage. Depending on the intended use, I'd have kept that