Mark,
as my friend Montenegrin said on the other board, this seems to be a conversion from ML to BL, circa 1868-1875.
To restore this sleeping beauty to its former shooting glory, a first rate gunsmith is required. If in your boots, I would contact Mr. Glanznig of Ferlach (Wilfried Glanznig, Werkstrasse 9, A-9170 Ferlach/Borovlje, Austria, Fax +4342274851); I'm sure there are master gunsmiths in USA that can do it properly, I just am not familiar with them.
First possibility is to line the modified barrel back to .577. Another possibility is to bore out the other barrel too, and then put .500 caliber liners to both barrels. This second approach would not be so accurate historicaly, and the barrels would be a bit heavier, but on the other hand you would gain on safety which is always good when working with old barrels of uncertain properties/condition. A third possibilty is to put .577 liners to both barrels. I would leave the final decision to the master gunsmith who would do it (but would cross my fingers in hope the third option would be acceptable).