ΔE 20.4, loose: different enough that you should pick on purpose. Leadbelcher is noticeably lighter than Rough Iron.
Can I use Leadbelcher instead of Rough Iron?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Leadbelcher is noticeably lighter than Rough Iron.
Can I use Rough Iron instead of Leadbelcher?
The same difference applies the other way round: Rough Iron is noticeably darker.
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.