As stated above, the tang safe M77 action was too short for the 375/404 length cartridges.
When Ruger introduced the Mkll Magnum circa '91, they did announce 404 as one of the initial chamberings. However, I'm 99.9% certain that they never made any, and having a little insight into S-R's corporate thinking I suspect I know why. They probably had a verbal agreement from one of the major ammo companies that 404 ammo would be available (Federal comes to mind, since they announced 416 Rig about the same time). When the promised ammo never materialized, they cancelled the 404 rifles, but not before a handful (< 100) of No 1s made it out of the plant.

This would coincide with long standing corporate policy to not build guns in any chambering for which ammo wasn't available from a MAJOR ammo company (read as big 3).

SOS