Dai S Loe,
Replied to your post at ASSRA site, and here is a copy:
"Might be a Dwight Chapin, Ball and Williams, or a R. Ball, but not a Merrimack if it was brought to NZ in 1866-67 time, as Merrimack didn't start until 1867 as a Ballard maker.
I don't know whether to be concerned or excited about the Spencer caliber? I have never heard of, or seen a Ballard chambered for that caliber, and it's either rebored, very rare, or not that caliber. There was a .54 Ballrd cartridge, which was really a .52 Sharps & Hankins. The vast majority of these era Ballards were .46, .44, or .38 rimfires.
John Dutcher tells of one that was used by Winchester Repeating Arms Co. that was marked ".56-56", (.56-56 Spencer) but was only made for testing their ammo, and never left the factory.
Look forward to hearing what maker is on your gun's barrel. I would strongly agree with Charlie on NOT modifying this gun's originality with restoration or permanent changes. It's seen far too much history to destroy it now with a complete restoration!"
In addition, all Spencer cartridges were rimfires, so ammo is not available today. Some old guns were converted to centerfire, (and so might your's) and special ammo made to shoot in these guns. Try Buffalo Arms for brass, or closer to you, Bertram brass to see if they make it. First confirm it is that caliber, second that it is ceneterfire.
Hope this helps,
Vall