just get the contessa picatinny rail for the 303. The OEM ISI mount is expensive for what it is.
The description of 'newer' 303s used to be copy + pasted (presumably from Sauer?) to say that it used the same mounts as the 404 (SUM). I have never seen one in Canada, and had reached out to the distributor a few years ago to confirm that they did NOT use the SUM. It would be neat if you could swap the QD from your 404 to your 303. The SUM mount actually works pretty well.
I had gotten a 404 pic rail from MDCharlton via Prophet River a few years back but returned it because it was an odd (and unwanted) 25 MOA.