The other option is to remove the safety spring from the sear bar. It is riveted on. I would encourage you to let someone with the correct tools remove it and then make sure that the side plate and firing train are properly adjusted.
Many of these safety modifications were removed by the Germans - it's not that unusual.
The original side plate will have a number related to the first 2 digits of the serial number stamped on the inside. It might be the same digits, or the first two digits plus 1. For your gun, I'd expect to find "61" or "62" stamped inside of the factory original side plate.
I don't own a Luger with this modification, so cannot guide you on fit. Are you sure that it doesn't fit. Perhaps someone can post the correct procedure for re-installing it. It's also possible that your relative acquired this side plate (with the same external sn "07") and was trying to fit it to the gun.
Marc
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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