Vintage Synth Repair

The tolerances of the Trident keys are very small.

The "hook" on the underside is actually smaller than the one on the upper
side. It will pass the metal frame but may require a bit more nudging. I
don't know if your metal frame has also the adhesive tape on top of it?
If you look at it very carefully you'll notice that the key simply cannot
move any more toward the front since it already hits the other side of the
metal frame. So this is the only way - pull it toward the fron as far as
possible and then somehow try to wiggle it out of the keybed.

The Trident keyboard is unlike most of the other keyboards from later era
Korg (Polysix and onwards) because it uses metal contacts. I believe the
MS-20 has a similar construction.