Springfield Armory refers to these as a "hook plate". They don't show the mags with them on their store page but they have them. I do not know if they sell the extension individually. You could call them and ask.
I have the same setup as described above, but found this magazine uncomfortable due to my XL hands. I ordered the 8-round magazine, which is much more comfortable and does not print as much as the 9-rounder.Since they don't seem to have a picture on their website, I'll post a few here of the finished product.
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You want to call them for sure. When I ordered mine, they told me to type in "with hook plate" in the instructions. That would give you exactly what you see in the pic above.
The gun pictured is an XDS Mod 2 in 9MM. The XDS and The XDS Mod 2 have different texturing on the mag extensions. I am not sure if the hook plate would matter. Definitely call them before they send out the shipment and explain to them what you have and what you are looking for. Customer Service has always been great to deal with.
800-680-6866