ΔE 16.7, loose: different enough that you should pick on purpose. Silver Fox is noticeably lighter than Leadbelcher.
Can I use Silver Fox instead of Leadbelcher?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Silver Fox is noticeably lighter than Leadbelcher.
Can I use Leadbelcher instead of Silver Fox?
The same difference applies the other way round: Leadbelcher 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.