Never seen Winchester or any US based companies producing 303/25.
Yes, Bertram did produce brass cases and at one time they were available as loaded rounds, whether these were loaded by Bertram himself or outsourced I do not know. Terry Warnock who is a member here, is the ‘go to’ man for Bertram products.
I have shot and collected 303/25 over the years, although most of my collection packets have been sold or traded. I have seen other proprietory brands such as Benz, Sniper, Sportco with 303/25, along of course with Riverbrand and Super. No doubt there were others.
If your inquiry is based around feeding a rifle, then do as I did and purchase a .243 Winchester!