From the depth of the fences I would believe it to be an original central fire, though the style is reminiscent of the pin-fire. The shape of the lock plate where it abuts the action is different from the only Reilly pin-fire I have, if that means anything (see below). I'll be interested in hearing what you get back from Southam's.

[Linked Image from i.imgur.com]
[Linked Image from i.imgur.com]
[Linked Image from i.imgur.com]