The disc was dropped on the Kriegsmodell stocks and a cross hole in the cup type buttplate substituted.
You don't say who the manufacturer was. It is possible that Mauser had some stocks left over from a foreign contract (Portugal?) and was allowed to use them up in K.98k production. That would be unusual at an early stage, but things like that did happen. Pics of the rifle markings, serial number and the stock serial number would help in figuring out what you have.
You might also check here
http://www.k98kforum.com/forum.php
where there are some folks who are pretty knowledgeable on Mausers.
Jim