My friend has a Marlin camp carbine, but his is in 45 caliber. The opening in his wooden stock looks just like what you have.
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On the other hand...I think you guessed it.
Thank you for all your help I went down the rabbit hole I do believe it is a camp 9, not sure of the caliber yet but it's enough information to post it for saleOn the other hand...I think you guessed it.