When, during the renovation of the Fürstengruft in 1951, the marble slabs that had been placed in front of the niches in 1893 had to be reattached, it was possible to take a look inside the graves. It was discovered that many graves had already been opened. Behind the slabs were walls of field-baked bricks, some of which were loose and allowed a view into the interior of the niches. In the light of a strong flashlight one could see that in the niche of Frederick William Adolf, at the foot of the very well preserved coffin, there is a box of about 60x40x40 cm, made of zinc or lead.Lück & Wunderlich (1956), p. 37–38.
