1858 Remington conversion ?

[Hewy]

When was the first metallic cartridge 1858 made ? Was it Remington or a outside gunsmith ?
From what I see from old photos, the rear of bored through cylider was a small thin plate , and a cartridge loading cutout
in the recoil sheild. Could the plate be  closed  in order not to prematurely eject cartridges that were not fired? or even fired
cases for that matter.