I saw your WTB ad ...someone had the same 4 position sight on a 44 mag barrel recently for sale...They did a 6" Lew Horton (had one,wish I'd kept it).
Check the for sale stuff,Guy has been selling a bunch of barrels lately and you could remove that one,think the price was around 130.
If you're building one ,an Arisocrat...or bomar,or others might have some options...I know the old PPC guns had an adjustable Front.
Jason