These are stock-piled backup KH-11 Kennan reconnaissance satellites, which also had the codename of
"Key Hole".
As I mentioned some time back in
this thread, the dimensions of the Shuttle, in particular the payload bay,
was specifically designed to accommodate them.
The Hubble was built by primarily the same contractors who engineered the KH-11.
Ironically the KH-11's ended up getting launched on Titans.
Like the Hubble, they had mirrors of around 2.4m in diameter but are reportedly f/8
rather than f/24 and have movable secondaries.