This is very curious. As I alluded to earlier, I have never seen the "dash" variation indicated other than on the same line, in the same font, with the correct spacing, and appearing as though it had all been stamped at the same time. If there's insufficient room in the frame cutout, the "Mod" part is usually on a different "line" immediately above the numerals. None of that is the case here, and I don't think that "2" is intended to indicate that the gun is a -2 model - at least I've never seen the dash variation marked that way. The "M" on the other hand appears identical to the M stamped on my guns to indicate that they had the firing pin/bushing modification made. The confusion comes because, according to SCSW, the SN should indicate 1988 production, at which time it should have been a -2, which didn't need the modification.
SNs have been used out of their correct order at times, and that may be the case here, so you may have a "no dash" with a SN out of normal range that was subsequently modified in response to the recall. Jim (laytonj1), if you're reading this perhaps you could venture an opinion - you know tons more about this stuff than I do.
Given the confusion, and the fact that this model was the subject of a very rare recall, I'm sure that S&W Customer Service would be willing and able to tell you exactly what the story is if you give them the SN. If you do call them and find out, please post back here and let us know what the answer is.