ΔE 20.7, loose: different enough that you should pick on purpose. Shadowed Steel is noticeably darker than Leadbelcher.
Can I use Shadowed Steel instead of Leadbelcher?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Shadowed Steel is noticeably darker than Leadbelcher.
Can I use Leadbelcher instead of Shadowed Steel?
The same difference applies the other way round: Leadbelcher is noticeably lighter.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.