Hey, sorry if this is a dumb question but I got these 2 batteries mixed up and I'm selling another n900 and I'd like to send the newest unused one with it. Which one is the most recent one? the serial number on the left starts with 49 and on the right with 46 so i was thinking the left one but they both look identical and have no date or anything. Thanks!

http://s2.kimag.es/share/7444266.jpg
http://s2.kimag.es/share/39837647.jpg

Charge both batteries until full, stick in N900 and use battery monitor to see which one holds the better charge.

does one look more worn out than the other? Does it have a few more scratches? That's probably the older one

I have a couple of 49s myself. I think you are probably right; they are newer.

I am probably wrong but there is might be a way to check capacity of of both batteries. My ubuntu shows laptop's battery capacity level, but the laptop's battery has more pins on it. So, there is probably a special interface that simply "tells" laptop the values.
Also, look at the pins. If they are starched, then it's probably your old battery

they both look identical, no scratches or anything, i guess i'll consider the 49 one the newest, but i'll try to do some tests