The closest I can get is this example... much later (1980's).

I would hazzard that it would be either Greek Key or Triclub mold - or a jeton.

Most UK chips from that period (1968 - 1971) fall into those holes

As to colour - your guess is as good as mine - but if it is GK or Triclub - odds slightly favour Grey / white as the replacement decimal 25p was grey (this rule is not even close to 50% true in all cases)

Hope this helps

Ross

PS If you ever have one... I'm a buyer... BIG STYLE!!!