N5xxxxx --
Per the SCS&W, that would have been made somewhere in the range of 1978 to 1980.
What digit comes after the 5? It could be narrowed down more if you gave a little more of the s/n.
Also be sure you give the right count of x's.
N5xxxx (N5 followed by 4 digits ) is 1970 to 1972 (probably closer to '71 / '72).
N5xxxxx (N5 followed by 5 digits as you show) is '78 to '80 so that's quite a difference.
Sorry I have no idea what the shield indicates. It looks like a very desireable 4" M28, so congrats on the acquisition!