It might have been defarbed (stripped of all markings). I recently sold an ASM 1851 .36 that had no markings, and I don't know it if was made that way or worked over.
It might be an original "lunchbox special", one that was removed from the factory by an employee. I recently saw (and almost bought) an 1862 Pocket Police that was definitely an original but had no markings on it at all.
Pics would help. Also, the nipple thread size and barrel twist.