Existing items do not offer mundane with +X
There are currently no official items that grant a +1 to either attack or damage that aren’t also magical.
That leaves us without guidance from existing material and whatever your source material you have is going to contain your answer. If this is homebrew (or if there is no stat block), then it is up to the DM to determine the statistics of the weapon.
It will depend on the weapon’s defined stat block
Whether it’s +1 to Attack, Damage, or Attack + Damage will be determined by the stat block of the weapon.
If you are using a published adventure or module, you’ll need to check the stat block for the weapon.