I forwarded the picture of unusual bolts to an elderly friend and airplane builder, who was a machinist before he retired. He responded that he once worked for a company that built Jails and they used a lot of bolts that had a double countersunk heads. The holes were countersunk extra deep too accept the first head, then torqued down until the second (top) head broke off, then welded and ground flush. This prevented the inmates from removing the bolts.
Jerry
Jerry