Serial number question

[Ireload2]

At the last guns show I ran into a nearly new .410 with a serial number prefix NN. From what I remembered in the FAQ I thought this was
a late model steel frame. However I was able to open it enough to shine my little Maglite under the barrel and it appeared it was a cast iron frame.
Did I misunderstand the serial number N was a SB2 frame?

[quickdtoo]

The first N = NEF, H = H&R, the second denotes year of mfr, N= 1999. The .410 would come from the factory on an untreated cast SB1 frame, the SB2 rifle frame is heat treated investment cast alloy.

Tim