Thoughts On The Authenticity On This Charles II Scottish 1 Merk

I can't see anything wrong with this, apart from the hole!
Where did you get the expected weight from? My book on Scottish coins gives a weight range for Merks 1664 to 1675 of 5.6 to 6.5g, so you are well within range. (1675 to 1682 is 6.4 to 6.6g.) Book is the 1999 Coincraft book on the coins of Scotland, Ireland, Channel Islands & Isle of Man.

They got silver refining up to a reasonable standard by the time this coin was minted, but not to the standard of what electrolytic refining can achieve.

Lettering and devices on reverse look a little 'mushy', but in this case, could be entirely due to wear.

Would XRF testing be of any help?
Perhaps take it to a scrap gold buyer, they usually have access to a hand held XRF tester. Look in the analysis for trace impurities and metals, not for the purity of the silver. If there is a discreet alloy of copper and silver only, it is probably fake, if there is a tiny amount of random 'rubbish' in the alloy, (well less than 1%), it is probably genuine.

Looks good to me (I see a few familiar die diagnostics) - weight diff likely as a result of the hole. These are notorious for being all over the place in terms of weight and composition.