I personally support the drain-hole theory......
 

..

+1!

Just kidding! ...though I wondered if it might explain the tapped hole in the grip strap of MY 1906 AE! Well, really, I don't see any purpose for your hole, beyond its being the evidence of a retro-fitted stock of some sort. I plan to TIG up the hole in mine, though I'll document the existing situation with pics, and save the knurled thumbscrew doo-hickey that was screwed into it, then spot-refinish. Mine has already been refinished, so doing this won't affect its collectability.

Hole
 

"Guys..ca'mon! Everyone knows this is an Imperial U-Boot Luger! The holes is in the bottom for when they come to the surface..to let the water out!"

That's silly. It's the tripod mount screw hole. ;)

Regards,

Gunny John

My guess for that hole is that she could have been a part of some sort of exhibit or a display in the past. Metal holder in approximate dimensions of the magazine would fit in mag well and with that screw gun would be locked in place so no one can remove it.