Added in the decal..and they look really out of place...but I guess there must be reasons for it to be so big with "an out of place color" or maybe it was peace time training in Germany ? Can those who serve during the "Exercise Reforger" time frame share the reason for the non tactical marking ?
The "decal" as you put it was actually a paper (early) or Vinyl (late) square panel that was issued to units before Reforger or other major exercises. It was commonly taped to the turret or vehicle sides and the front of the glacis, hood or doors. The number identified a numerical specific designation for the unit (other than the units actual designation), and the color of the panel designated the unit as either a member of the "blue" or "orange" army. These were used by umpires and controllers to "officiate" the training. Umpire vehicles used a white panel as I recall (but it's been a while-- so my memory may be a bit faulty here). Depending on where in the world you were and what exercise you might be in, there have been variations in panel sizes and types (triangles and circles have also been used in other locations outside Europe). These number "boxes" were prepared and issued through the Training Aid Support Center (TASC) at the installation or unit level. They were supposed to be collected up after the exercise for future use, but were often just tossed (or kept as a souvenir). Typically, they were attached to the vehicle with "100 Mile an hour" tape --what we know today as OD green duct tape. In later years, strips of Velcro tape have been used to attach both the MILES gear (a laser training aid targeting system) and these panels. "513" would be an identifier for some armored unit (either at company, Troop, Squadron or or Battalion level, and would be used to make assessments in attrition and combat strength during an exercise. Umpires and exercise controllers would keep corresponding score sheets as unit controllers would assess the effects of training simulated action or "events" for opposing units. One common trait of using this "stick on system was that the adhesive on the 100 MPH tape would stick to the vehicle long after the panel was removed, leaving an odd square or rectangle of sticky residue which attracted dirt-- making the area rather distinctive. Hope this helps explain it.