That would be tough to place an outright value on. It seems that any post WW1 markings generally devalue a pistol to a minor degree. In other words, an Erfurt artillery LP08 will sell for more typically if the '1920' property mark wasn't there. However, the flip side of that is that Navy anything typically adds a premium to a pistol. For example, a matching 1936 Mauser luger will fetch a certain price range, but add a period Navy marking to it, and that adds to the value...sometimes significantly.
This pistol is an odd, but seemingly legitimate combination of the two. Erfurt artillery lugers have been selling for right around $3k...some higher, some lower depending on condition.
Here's a nice one that recently sold. Probably $3300 or so after shipping & taxes.
https://www.gunbroker.com/item/986056494
I like odd pistols such as this and would pay up for it. Matching magazine (if legitimate) is also a nice plus. I would say that's easily a $3500 or better gun to the right buyer.