A fine rifle sir, and thanks for posting, along with the details.
I question whether Leman or any apprentice would stock and sign a rifle as an apprentice - i think the master would maintain control. That does not take away from this rifle. I agree it's probably a very early Leman, possibly in 1837 when he was on his own. There are also some Leman rifles with 36 - 40" barrels, the horsehead patchbox, his typical later mounts but the trigger guard spur curved to the rear. Some feel these may be from his earliest contracts.
I've seen folks suggest that a lesser rifle may have been made by "so and so" while an apprentice, before he had the skills of his master. Again i question whether any such work left a master's shop.
The mounts, including the engraved sideplate may have come from M Fordney or others, and the sideplate may have been engraved before it was put on the rifle.
Much to study and enjoy, thanks, Bob