A 105N material is supplied under an ASTM specification, while SA 105N material is supplied under an ASME specification. There are no differences between these two specifications (ASME SA 105-98).
ASME SA 105 (in the 2007 ASME Section IIA) is identical to ASTM A 105-2003. The current version of ASTM A105 is 2005. The only change that has occurred since the 2003 version is:
Revised 12.2.2 to require reporting the width of the gage length of longitudinal strip tensile specimens, if they are used.
If this change does not affect you, then the two are the same. The ASME version usually lags the ASTM version.