Marking at Hexagonal Shaft

Dimension across flats for the nuts or bolt heads for which it can be used.

nn is right. It is called (at least at our part) the pipe box spanner or box spanner.

So one is a/c flat 12 on one end and 13 on other (ie for M8 screws/nuts) the other 16/17 is clearly for M10.

Also spark plug wrenches look like that.

They are so called "Box" spanners/wrenches.

The numbers marked/stamped on the side are the sizes in millimeters of the nut it will fit (in this case. They were/are also made in many other formats - AF, Whitworth etc.) .

If you go here:-

http://en.wikipedia.org/wiki/Box_wrench

You will find only one line mentioning Box spanners/wrenches, but here it is:-

Box spanner: a tube with six-sided sockets on both ends. It is turned with a short length of rod (tommy bar or T bar) inserted through two holes in the middle of the tube.

They were popular in the UK at least, after the 2nd World War when money was very short as they were far cheaper to buy than a full set of proper "sockets". Sometimes the length is a problem when space is restricted, we would then cut off the end we wanted, adding holes with a drill for turning with the "Tommy Bar"....and buy a replacement when the shops opened......they were that cheap.

Ones for spark plugs may have a "rubber grommet" fitted to hold the top of the plug once its undone to prevent it falling out.....useful on engines with socketed/recessed spark plugs.