I don't think there is a stock bolt under the plug holding the stock to the receiver. Believe you can remove the stock at the tang screws....but am totally unsure if that is the case on an AYA box lock. We had one, a 28 ga 453 that was my wife's gun many years ago...about 20 if I remember right. The single trigger caused much trouble. Good to see yours has twin triggers. Nice shotgun, good luck with your project!
Here's my previous post for taking down an Anson & Deeley boxlock, and your gun is probably another A&D style gun so this should apply. This is also why Guru suggested a smith for takedown and assembly.
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I assume you have a padded vise, gunsmith quality screwdrivers, and skills/patience to not bugger the screws.
Remove the Triggerguard
* Remove wood screws
* Unscrew the triggerguard, the threaded stud is at the front of the t-guard
Remove the Bottomplate
* Remove the big screw on the bottom of the action holding the Bottomplate
* Tap the bottomplate out from above with a wood dowel
Remove the Triggerplate
* Loosen the Breech Screw (under the opening lever) 1/2 a turn - careful this can be an MF'er
* Turn the gun over, and loosen the two screws on the Triggerplate
* Turn the gun over again and remove the Breech Screw
* Over again and remove the trigger plate screws, the back one goes through the stock.
* Remove the Triggerplate
Now you can pull the stock.
I copied this off Shotgun World, and believe it may apply to your gun but am not sure.