I do not know how a palimpsest could be justified on a frieze.

[link to en.wikipedia.org]

"A palimpsest is a manuscript page, whether from scroll or book that has been written on, scraped off, and used again. The word palimpsest comes through Latin from two Greek roots (palin + psEn) meaning "scraped again." Romans wrote on wax-coated tablets that could be reused, and a passing use of the rather bookish term "palimpsest" by Cicero seems to refer to this practice."

"The word palimpsest also refers to a plaque which has been turned around and engraved on what was originally the back."

Either way, to me, the intent of the engraver seems to be original. Especially the helicopter.