I've been on the lookout for a vintage Sheaffer Flat-top or OS Balance with a flex nib for a number of years. I prefer to find one in the wild or on e-bay to avoid paying retail for a scarce nib. Last week the looking paid off. I managed to win an OS Black Balance lever filler on e-bay and when it arrived, I discovered the flex nib. Woo Hoo!
I thought the nib looked a bit different than normal but the pictures weren't definitive and it seemed pointless to ask - so I put in a very reasonable bid and now I have a very cool pen.

Oh yea - all it needed was a good buffing and a new sac - along with the normal good flushing with any vintage pen.

I'm a happy guy.