The breechplug garbage mentioned above is about it.
There is no functional, working definition.
I am not sure that we would want the powers that be to come up with a carved in stone interpretation.
The op asks if swapping out the reproduction lock on an antique caplock rifle for an antique lock makes the thing antique.
I don't really understand the question. This is a prescribed antique rifle which has been fitted with a modern lock? Can't see that it wouldn't still be a prescribed antique, even with a modern lock.
A modern made caplock rifle is a n-r firearm. Installing an antique lock isn't going to make it antique.
Here are some questions:
This is an antique NOS lock.

If it is assembled into a handgun using a pre-1898 barrel is the resulting gun a prescribed antique?
If it is assembled into a handgun using a modern barrel is the resulting gun a prescribed antique?
Pre-1898 breechplug in a modern barrel?
Would it make a difference if the chunk of wood used to carve the stock was pre-1898?
Can of worms.
Some years ago there was a case where a pre-1898 muzzleloading handgun was repaired using modern screws, etc. The authorities claimed that this resulted in loss of antique status, and the thing was a modern handgun. This claim was eventually rejected.