cupboard door corners

not sure if in right forum but have some panel cupboard doors want to re use will be painted so appearance is not a issue
they are just a pine edge with a recess groove in it so the wood panel slips into it
but is held together with little right angle joiners at the corners (havent seen these before ) 3 per corner appear to be nailed into the frame

so
a. how do i remove them as need to reduce the doors down without destroying the door ?
b can you buy them as look like a good way of joining the corners , and if you can any tricks to using them ?


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