Apostle spoons are usually silver or pewter with an image of an apostle or other Christian religious figure as the termination of the handle, each bearing his distinctive emblem. The spoons were particularly popular in Pre-Reformation times when belief in the services of a patron saint was still strong. They symbolize the Last Supper of Christ in the company of the Apostles. "Bill" Shakespeare refers to them in Henry VIII and they’re mentioned by other dramatists of the period - plus there's a set in the British Museum from the 1530s. So Bellairs was probably familiar with them from those sources.
Oh. Saint Thomas is distinguishable from the others by the spear that he holds.