wogpotter,
I can completely understand your frustration concerning your OP in the thread you had started. I skimmed through it quickly. Clearly it was a good move on your part to ask question specific to the BATF to eliminate the noise concerning local law.
Some of the responses to this question, which gets asked often, just aren't logical but are rather based on the individuals perception of how things "should be" or perhaps based on their local laws...which, very smartly, you didn't ask about.
Those who state that a modified cap-n-ball revolver (to accept a conversion cylinder) "must", by law, be registered should help the rest of us understand what they believe the process is. For example, my questions to them would be:
1. Who do I register this modified firearm with?
2. What paperwork or forms (form number) do I need to fill out?
I am in possession of such a firearm. Specifically, I have a replica Walker that I had modified to allow breech loading by use of a gated Kirst conversion cylinder. Have I broken the law by not "registering" it? The gunsmith who did the modification and installed the cylinder shipped it directly to my house from another state. Did he break the law? Was he supposed to send it back to me only through a local FFL?