Also a really good “Easily Visible” Level/Anti-Cant Device that works well on the Top Pic Rail of the B.O. S.L.I.C.K. (Expensive, but works well) is the KAC Anti-Cant Device that can be found, at good KAC Dealers. (At least for now):
https://www.robertrtg.com/store/pc/viewPrd.asp?idproduct=8499...