Must have the main apostrophe to mark the omission of digits but not the pluralizing apostrophe). nowadays, partly as a consequence of being a programmer) do take the apostrophe when forming the plural. Examples that generally corroborate this, taken from the useless-tree American Heritage Dictionary Another thing must be mentioned. https://getsocialpr.com/story21419873/the-smart-trick-of-vehicle-marketplace-uk-that-no-one-is-discussing