Counterfeit PCGS Slab On 1816 British Shilling?

You cannot confirm the weight of a coin inside a slab.
You can't even check a slabbed coin with XRF to confirm authenticity.
If you have any doubt at all about the slab, then you should give the coin it a miss, even if your gut feeling says that the coin is genuine, because the coin inside can't be checked.

That is a pity.

I see nothing wrong with the slab pictured,other than it is scuffed up. And it is of a generation earlier than when the fake slabs first began appearing. As a rule the tend to fake the current or immediately preceeding generation of slab. I haven't seen fakes of this generation.

I have been fooled a few times buy buying stabbed coins by reputable TPGs, the seller has cracked the case just enough to get the coin out and swap it with either a cleaned coin or they have xeroxed the label and slapped a less valuable coin in a counterfeit case with a pretty realistic hologram. The truth always reveals itself when you get the coin and look it over, cracked slabs no matter how good still show signs of being opened, them counterfeit slabs however are tricky and one has to be meticulous when looking them over. The counterfeiters I have run into prefer older graded coins which are easier to fool you with on the label, but if you ever have a doubt always contact the TPG and verify, scammers are getting really good nowadays and it's up to all of us to stay vigilant and keep them from ruining our hobby and in some cases our livelihood as dropping hundreds if not thousands on a coin only to realize you have been had after its too late can really put you off