Lock question on mills 47N

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Does anyone have any experience with the side door on the mills 47N ? I have had three locksmiths look at it and they had no clue. I'm not sure if driilling out the lock is the only thing I need to do. In the picture below there is a hole above the key hole that looks like there's a screw mechanism to unlock it similar to the allen on a jacobs 56. Its not an allen though. Looking in the hole it looks triangular. I'm just not sure of my best option. Any help would be appreciated.