Bayonet shifter - won't give!

Hello everyone,
I read that to remove a bayonet shifter, a .01" feeler guage works. I've been trying for a while now and the guage gets in about 1/4 inch and then stops. Attempts to push it further results in the guage bending from the force required. At this depth I've pulled and yanked and cursed up a storm but it's not budging.
I've used penatrating oil to help but still no difference.

Can someone tell me how far down the guage needs to go to push in the retainer to free the handle? Is the 1/4 inch enough and it's just stuck? Or do I need to use something stronger, to shove it deeper, along the shifter?

I am doing this on the drivers side and am sliding the guage down against the shifter itself, so I believe from all I've read that is the spot.
Any advice is welcomed, as this thing is kicking my butt!
TIA