Found a couple of Plombs at the flea market, a Combo 5/16 Pebble 1210, and a 1126 1/2-7/16 DBE.

Now for the odd ball.

It's a 5/16, 1/4 open end with a 1/4 6 point box stamped in the middle, and two copper feeler gauges riveted on the 5/16 end. The feeler gauges are marked "For Magnetoes" and "Sparkplugs".   There is a logo on the back that I think is the rounded H-B.
Herbrand? But there is a figure in the middle that's too small to make out.  It also could be an A-B (American Bosch?)

Any idea, what magneto this might have been used for. 

I know some pic's would help.   I'll work on it.

Probably Bosch. Lots of them in ignition kits.

Can't help, but I used to have a similar wrench for a CZ/Jawa (Czech) motorcycle.  Motorcycle tool kits tend to concentrate as much function in a given tool as possible, since there's not a lot of room for a big toolbox on a motorcycle.

I would also say american bosch.  Especially if this is the logo.

http://www.ebay.com/itm/American-Bosch-Magneto-Corp-Wrench-with-Feeler-Gauge-/292272417837?hash=item440ccb2c2d:g:4AoAAOSwSYpZy9OZ

Thanks for the pic of the AB logo.  That's it!  It's stamped in and you need a good magnifying glass to make out the details.
In my opinion, not a bad buy for a quarter,

Now you just need to find a vehicle with the appropriate magneto to use it on!

Got one, it's called a Cessna.

Thanks all of you for your help.