I haven’t had one in a Contender, but have in a Hawkeye. I’ve strongly considered building one on a GP100 as well. H110/W296 has been my go-to powder for the 256, just like any other magnum rimmed cartridge round. With the light 75’s, you won’t be heavy enough, I don’t believe, to warrant lil’ gun, so in your shoes, I’d keep my focus on H110/W296.

In a Contender, less constrained by tip length, I would spend my time with the 75 V max over H110/W296 or H4227. From my Hawkeye, scoped and benched I generally saw about 2.5-3” at 100yrds as my standard for that pistol. But that’s saying more about the platform than the cartridge or bullet - a Contender with an appropriate load, scoped and benched, would beat that with ease.

It’s an obscure round, and the platforms in which it was offered were extremely varied - leverguns, singleshot break actions, and singleshot rotating breechblock pistols (built on a revolver frame, no less)... what worked well for my Hawkeye would have no bearing on what will work well in your Contender, other than to confirm, for magnum revolver cartridges, it’s difficult to beat W296/H110, H4227, and 2400 as top contenders - backed up by the fact they remained to work so well in my Hawkeye in 256. Seems like you don’t have much to complain about with your 19.0 grn load, however!