No, you shouldn't! The rear sight is dovetailed into the mounting-plate and the whole sight/plate combination is removed exactly as shown in the linked video. The sight itself should be left as it is on the mounting-plate.Should I remove the rear sight before removing the mount base?
The only thing that I can think of that might be causing the resistance is that maybe some loctite extruded into the space between the plate and the slide when the screws were inserted at the factory. If the mounting-plate or slide-cut were out of spec, then they wouldn't have got the plate on in the first place.
Try bumping the plate with a plastic or wooden mallet.
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