Here is weight data on most of my William III Halfpence. Most measure around 27.7mm to a 28.6mm in worn condition. A 1 1/8 (roughly 28.5mm )measurement The tell all would be weight, with a copper that size and thickness my feeling is if it would fall within the parameters of the weights of mine, especially the well worn ones I have than a stronger possibility it is a William copper, but if much lighter, than a KGIII counterfeit.
125 grains =8.1 grams(low end) 155 grains = 10 grams(High end) ( a small postal scale would measure grams, there are conversion programs on the web, but if your worn out copper falls between 8-10 grams, then we could eliminate a KGIII worn out copper)
1694 William& Mary 125.9 gns
1694 William & Mary 124.2 gns
1695 William III 154.4 gns
1697 William III 132.4 gns
1699 William III 132.3 gns
ND William III 104.9 gns (counterfeit?)
ND William III 124.2 gns
ND William III 133.8 gns
ND William III 130.8 gns
ND William III 125.8 gns
ND William III 134.6 gns
ND William III 148.1 gns
ND William III 151.8 gns
ND William III 137.0 gns
ND William III 73.5 gns (Cut in Half)
ND William III 131.9 gns
ND William III 125.7 gns
ND William III 145.4 gns
ND William III 110.5 gns(counterfeit?)
Don