Directly from the manufacturers (was PoA Chapter 14 Summary)

[Rita Winston]

No: HPFGUIDX 20533

Rosmerta's text that I am replying to is below my answer because it is 
long:

I believe that Snape doesn't know the nicknames Moony, Wormtail, 
Padfoot, and Prongs. But when the parchment insulted him, he 
recognized the Very Same Phrases that Potter, Black & Co had insulted 
him with back in schooldays. This made him suspect that Potter, Black 
& Co were involved in making the magic parchment, which made him 
suspect that Lupin, as the only one of 'em left, had given Harry the 
parchment. Snape had nothing to go on but a suspicion, a hunch, so he 
tried to trick Lupin into confessing. The "manufacturers" line was an 
attempt to bluff Lupin that Snape knew more than he really did. But 
Lupin out-tricked him and avoided telling him anything.

So Snape is left without nothing but his suspicion, no evidence, no 
confession, and Dumbledore has repeatedly made it clear (altho' we 
only SEE him doing so after the Fat Lady was slashed) that he doesn't 
want to hear any mere suspicions against Lupin from Snape. 

--- In HPforGrownups at y..., "Rosmerta" <tmayor at m...> wrote:
 
> If Snape *does* know who MWPP are 1) why call Lupin? The second 
> Snape sees their names appear under the very specific insults 
> directed his way, it would be obvious (if he knew who they were) 
> that the parchment (he doesn't yet know it's a map) is some 
> artifact left over from their time at Hogwarts. Why call Remus? 
> 
> 2) (Snip) So, by process of elimination, is Snape 
> really asking Lupin, "did you give this to Harry?" 
> 
> But if that is what Snape's asking--if he knows who MWPP are in 
> other words--it seems completely out of character (and also 
> irresponsible as a teacher and official Hogwarts gadfly) that
> Snape would let the momentary noise of Ron bursting in and the 
> extremely mild blustering of Lupin distract him enough to let them
> all leave his office scott-free and with the map in tow.
> 
> If you remove that one sentence (snip) everything else Snape says
> (snip) seems to indicate that he of course knows who James etc. 
> were but *doesn't* know them by their nicknames, Moony Wormtail 
> Padfoot and Prongs. In which case, what the heck *did* Snape mean
> with his "manufacturers" line?