There are also sketches and pencil tests of several pieces, which reveal much of Escher’s design process.
Updated my analysis with some additional NSA #FOIA ID's contributed by other searchers, it appears that the agency might receive an average of 6.5 requests per day over the past five years.
I believe the NSA might have a German Tank problem with the FOIA queue: the request identifiers appear to be monotonic integers, so my linear regression estimates that they receive around six requests per day.
#FOIA time! Or at least an acknowledgement of receipt... my other NSA FOIA request is going to be celebrating its third birthday soon.
I'm also not certain if I can allow anyone else to sit in my zelfgemaakt copy of Rietveld's Red/Blue chair.