Steve, Welcome to the forum.
Sorry for the news you've received, but it's important to be informed.
As Ron mentions, it is not legal to own a Luger and a non-appropriate (original or reproduction) stock. The ATF classifies this combination as a Short Barreled Rifle (SBR) and this is considered a NFA Class III firearm which requires special registration and tax stamp transfers.
Artillery Lugers with Artillery Stocks (original or reproduction) and Navy Lugers with Navy Stocks (original or reproduction) and a few rarer combinations are specifically exempted from this through a curio and relic list published by the ATF.
Unfortunately the combination of this stock attached to any Luger would be a SBR. Some speculate that the possession of both objects constitutes "intent" just like owning full auto parts that would fit a AK47 you also owned.
Marc
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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