I always thought that Hubble in fact was a declassified KH-11. And indeed the NRO declassified and donated a couple more obsolete Keyholes to NASA in 2012 [1], one of which is now being used as the chassis of the NGRST [2].
Well, it wasn’t. This is pretty well documented. It has pretty much the same design requirements and even some of the same contractors, but it also differed in key ways. It had a totally different instrumentation board, optical packages, and was built for servicing which KH wasn’t.
The fact that it was the same form factor is not an accident, as Hubble was sized to fit in the shuttle payload bay, which was itself sized based on keyhole.
[1] https://en.wikipedia.org/wiki/2012_National_Reconnaissance_O...
[2] https://en.wikipedia.org/wiki/Nancy_Grace_Roman_Space_Telesc...
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