I dug up a few of these metal detecting in Germany when I was stationed there. They can be difficult to decipher. Yours looks to be aluminum which I think makes it later in the war. Pz stands for Panzer (tank) and the Abt can mean section, detachment, battalion, etc. Unlike US tags there is no owners name. The tag would be issued upon entry into the military (Herr) by the soldiers first unit, in this case the Pz. Jnst. Abt. 551. The 6 is a roster number which was recorded in the issuing units records along with name and other things. Another thing that makes these confusing is a soldier kept the tag when reassigned to a new unit, unless they lost it. In that case the unit they were in would issue one with their unit designation. No idea what the Jnst means. Here is a link to German military abbreviations printed by the US military in 1943. It is not in there. Maybe it was not in use yet or maybe we just did not know about it.

http://usacac.army.mil/cac2/cgsc/carl/wwIIspec/number12.pdf

It does not have a blood type stamped on it so it was maybe never issued. I found most of mine in what was a POW holding area near the end of the war. I offered to give them to a local historical society. They did not want the tags but were very interested in the info which they said the were going to pass on to some other group. Maybe member fidbald will see this I think he knows more than I do.

or Instruktor.

might be that it was shoddily made in 1944 when Himmler collected all kinds of people for the Ersatzheer.

it's missing the bloodgroup.

there was a training camp "Truppenübungsplatz Thorn", today in the midst of Poland, then Prussia, where youngsters and old men and rests of other regiments were collected and then thrown against the Russians.