I only tested it on a 1k model, I couldn't downgrade to 6.60 from 6.61 (maybe corrupt updater), but I was able to downgrade to 5.00 from 6.61, 6.20+ might work, haven't tested it though.
Downgrader_661.zip
Update:
Issue with 6.61 -> 6.60 should now be fixed, tested and working on a 1k.
Chronoswitch_6.0.zip
Update2:
Now supports PSP 11g (E-1000/Street) from 6.61 -> 6.60, thanks to nzgamer41 for testing.
Chronoswitch_6.1.zip
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