It looks like your description is backwards."This is a new OEM Sig Sauer P250/P320 17-Round 9mm magazine with X-Grip Adapter. By sliding off the X-Grip adapter this magazine will also fit onto a Full Size P250 or P320"This reads like its a compact magazine that you adapt to fit into a full size, but isn't it a full sized magazine that you can use the adapter to fit into a compact frame?
Referenced product: MAG-MOD-C-9-17-X
Sig Sauer stopped offering this magazine but at the time it was marketed as a Compact magazine. It was indeed a 17rd magazine with X-Grip adapter making it fit a Compact P250 / P320. If you removed the X-Grip adapter, you essentially had a MAG-MOD-F-9-17 (full size mag) which fit a full size P320 or P250.