Funny note to add;
I had 6.60 PRO-C on my PSP-3000 before this, and updated to OFW 6.61 to swap to PRO-C2 and retain PlayStation Network accessibility on the OFW. Though, despite uninstalling PRO-C first, the CFW displays itself as being 6.61 PRO-C rather than 6.61 PRO-C2.
It isn't a big deal...