Bock's record of the war years was published in 1985 as Untergetaucht unter Freunden: ein Bericht. Amsterdam 1942-1945 ("Underground Among Friends: a report. Amsterdam 1942-1945"). The memoir tells how he disappeared from the official records by feigning suicide, which left him free to go underground, chiefly with the help of the German writer Wolfgang Frommel, a charismatic figure in the resistance group associated with the George circle. Frommel both arranged the practicalities of survival, and inspired the extraordinary literary activity by which those in hiding filled their days.
Frommel hid Claus and others in the house of his friend, the Dutch painter Gisèle van Waterschoot van der Gracht, on the Herengracht in the very centre of Amsterdam. In a chilling episode Bock recounted, the place was searched. A German officer detected a clear sign of the stowaways, but then, after exchanging glances with Frommel, promptly ordered his men to leave. The situation was depicted by another friend who took refuge in Holland, the German Expressionist painter Max Beckmann. In Beckmann's war-time triptych Actors (1941-42), the left-hand panel recalls the scene: Frommel confronts a helmeted soldier; Giséle hovers in the background; and beneath the floorboards or stage, the feet of some hidden boys appear, including those of the young Claus Bock.
I think I heard that Bock was hiding in a piano when a German officer looked him in the eye, but maybe my memory is playing me tricks. I most certainly didn't know this was referred to in a Beckmann painting.