The roundels are wrong.

His name is John, not Johns.

Therefore, the apostrophe HAS to go between the n and the s. Any other
signage is the product of illiterate designers.

JOHN'S

MARTIN'S




In the case of plural nouns, the apostrophe always goes afterwards. So

SPANIARDS' INN
if it refers to more than one Spaniard, or else

SPANIARD'S INN


Where there is some discussion is if the given name already ends with S.

Eg JAMES'S PALACE
or JAMES' PALACE

But never JAMES PALACE



The official line is, certainly as far as the University of London is
concerned where I did my studies and I am a sessional lecturer:

If it's Greek, it's always S'

If it's not, it's up to the individual as long as he or she is
consistent throughout.

Therefore, always Achilles' heel, Eros' statue, Nikolaidis' penalty
shot, Stavros' kebab house, Bacchus' wine, Androcles' lion, but if it's
not Greek, you can say James's Square or James' Square as long as it's
consistent throughout.