MSN V. FSN
 

What's the difference?

One's got a "M" and t'other's got a "F"? /sarcasm

You might want to provide a bit of context, as the acronyms probably have multiple meanings anyway. (Fox Sports News is FSN, but I doubt that's the answer you want, for example)

If you mean MSN as in Manufacturers serial number and FSN as in Fuselage serial number, they are the same thing.
However if MSN and FSN mean something else then ?

@dixi188 - provided they didn't get them mixed up on the production line.

I seem to recall at least one Hawk on the line at Kingston/Dunsold had more than one serial number ...

in fact, I just dug out a yellowing sheet of paper (a copy of an old magazine article in fact) which says front fuselage #264 was mated to centre/rear 241, as 241's "proper" front had been robbed for another aircraft.

But I'll be shocked if this was a question about HS works order numbers!