What transmission for 2005 "Classic" (old style body w/ 4cyl)?

I have a 2005 "Classic". Not the newer (from 2004 on), but the same body as the one re-introduced in '99 (I beleive), but with a 4 cyl engine (that maybe the same one as the 2004 Malibu replacement). The service manual calls it a "NP" model.

How do I find out the transmission model number? Do you guys need the VIN?

Thanks in advance. :smile:

BTW, the problem I just started to have (80k miles) it's slipping (shift seems to slightly 'pause') between 2nd & 3rd around 30 mph.