Can anyone identify these paving stones?

These were salvaged and quite likely discontinued, but I'd like to get more that match if possible - problem is I don't know what the type is called so I can't search for suppliers. If anyone knows, and has any recommendations on where to buy, it's much appreciated.

Pavers are roughly 1' square and an inch thick with - and this is the feature I can't match anywhere - half inch deep 'ledges' on alternating sides that allow you to lay them in an interlocking fashion without needing mortar. The sample in the picture has a broken corner, but otherwise should give (I hope) an idea of what I'm dealing with.