TC, from my limited experience, but to import a post 1900 gun requires an importer and believe it will be import marked. There is money involved and lots of paperwork. Bob Simpson's does this kind of imorting, you can find them in the Links and Resources section to your left, provided you came in at
www.lugerforum.com
The Luger you mention from your son-in-law could be one of about 300 variations of Lugers made from the early 1900's through 1945 not counting the newer 1970's Mausers and stainless lugers made now). Much more information is required to identify it, the grips are after the war or during the war modification, done by GI's or to sell the gun easier to GI's.
Explore the site a bit and use the search feature, there is tons of info avaialbe here for your reading pleasure, <img border="0" title="" alt="[Wink]" src="wink.gif" />
Welcome to the forum!
Ed