Beseler Negatrans: How is the 645 adapter removed?

My negatrans came with the 645 adapter and I'd like to replace that adapter with the standard (6x9?) adapter.

I see the locking tab, but how do you get the blasted adapter out? I don't want to break the glass....
Thanks for any help!
bob.

Got it. Just takes more brute force than I would have expected.