From what I can find so far, yes I suspect you are correct. I have seen a few of "N" guns now but most if not all were also stamped "Fultons" This one does not appear to have been, so my thought that Fultons used "N" is probably now binned. I have also seen "A" and just 6 digits aka ParkerHale and so marked.
---------- Post added at 06:42 PM ---------- Previous post was at 06:37 PM ----------
It is the bit the N XXXX is stamped on, the receiver. Under the rhs there will probably also be a F5X about 1/8th inch high which will tell you the year the receiver was made. On the RHS there will probably not be the un-machines single shot boss which suggests a late receiver. The F56 barrel could be an original or a replacement, as Ive seen a few F56 barrels on older guns. I assume they made a lot of barrels in 1956?