Ed
The problem arises only if you own a stock and a short barreled Luger with a stock lug and do not own a Luger that is appropriate for the stock. For example, if you have a Navy stock and a Navy Luger, you can only get in trouble by actually attaching the stock to any non-Navy Luger. You can have all the other Lugers you want, just don't attach a stock to the wrong one.
However, if you have a stock and
only short barrel Lugers (or the wrong long one, e.g. an lP.08 and a Navy stock) then the ATF can go after you. In that case your stock could only be used to create a short-barreled rifle. Of course, you can always register a short barrel luger, pay the $200 and your stock can be used with it legally. But it's the pistol that gets registered, not the stock, so you still can't use the stock on other ones.
Here it is, straight from the ATF:
https://www.atf.gov/firearms/qa/if-p...possession-nfa
They note that Lugers with stocks have been removed from the NFA, but that only applies to Lugers using the
correct stock.