At the 2014 Las Vegas Antique Arms Show there was a 12ga. Grade 3 toplever hammergun converted to hammerless that I and others had seen and handled. The ask on it was less $4K. Here's a link to that discussion and there's a link within this thread that takes you to another thread that includes a couple of pics taken by PML. I am not sure if it is the exact same gun discussed in both threads.
http://parkerguns.org/forums/showthr...ight=hammergun