This is a repro of the Walker nipple wrench/vise/screwdriver, and a pic of how it was used. From what I read, they weren't popular as mainspring vises.
Cabelas sells them and I'm sure Dixie Gunworks, Midway, etc. do, too.
I use the mainspring vise in the 3rd pic, but other shooters use visegrips with leather, plastic ties or just your hand.
Remember, the wedge enters from the opposite side compared to other Colts.
I use 45 - 50 grs of black powder, .454 balls, #11 caps, and it's a crowd pleaser.