That's an Abus Bordo Granit X-Plus 6500 white folding lock. The French site xxcycle.com was selling them in twin sets for a decent price even when considering paying overseas shipping. Their twin sets appear to be temporarily out of stock right now. The advantage of buying a twin set is they come with the same keys. In any event, I have a spare which I don't use unless I need the extra length for combining two Abuses together, or I'm parking in a less secure area where I think I need both locks. You can also buy an extra holster for this lock. They come in different colored Xs on the front, so for example you could combine a white or black Abus folding lock with a "red X" holster for a nice effect on a 2015 ST2. I'm sticking with my white X version for now.
To mount this particular lock to the water bottle bosses, I used two metric stainless steel button head cap screws, size M5-0.80 x 20. I also added nylon washers as spacers between the down tube and the holster because the Abus Bordo cage has curved ridges designed to mount to a round frame, and as you know the Stromer down tube is relatively flat. The right metric threading for the water bottle mount screws is critical or you risk stripping the threads, so be careful to buy the right ones. Also, since the screws are going into the frame close to the battery compartment, they can't be too long. The M5-0.80 x 20 size takes into account the nylon washers. The original button head cap screws on your Stromer are fine for water bottle cages, but way too short for mounting the 6500 holster. I was assured by Stromer that the weight of the lock wouldn't be a problem, with further assurances coming via internet photos of ST2s from European owners. They appear to be a favorite lock for this bike on the continent.
Of course, this means I have to quench my thirst from a location other than my down tube. It gives me an excuse to ride down to the local convenience store.