Most automatic pistols of the early 20th century were offered with shoulder stock options, and the Luger had more than most. Probably the most interesting one I am aware of is the Ideal Holster Company’s design, which was patented by one Ross Phillips of Los Angeles. Phillips applied for his patent in 1901, and had it granted in 1904. The design uses a set of special grip panels with metal locking surfaces in them in conjunction with some very clever angular geometry to allow the stock to be easily attached and detached when in the short position, but lock tight and secure when the stock is extended to shooting length.
Not many of these Ideal stocks exist today, and it seems that the idea was not commercially viable – or at least it was too expensive to become popular. All of the known examples are marked “Patents Pending”, which would suggest that all manufacture predates 1904, when the patent was granted. Most likely only one batch was made, and they took too long to sell to be deemed worth continuing to manufacture. The company did also offer this type of stock for colt and S&W revolvers, although those are also very scarce today.
Note that the pistol in this video is being sold by Rock Island as a separate lot from the stock, and the stock does come with a set of the requisite special grip panels.