This thread is old, but i stumbled across it while i was trying to find out the same information. I figured i'd share my research results in case someone else ends up here too.
While M, Z, and 3L belts are technically different, they are similar enough that they could be used in place of each other. The subtle size differences end up affecting the minimum pulley size and the top speed ratings. (thicker belts are faster, but can't do as tight of a radius) They're all likely to work on your drill press, but check the package and compare to your machine's specs as a double-check.
M belts:
Widest side: 10mm (.393 inches)
Height: 5.5mm (.216 inches)
Angle: 40°
3L belts:
Widest side: 3/8" (.375 inches)
Height: 7/32" (.218 inches)
Angle: 40°
Z Belts:
Widest side: 10mm (.393 inches)
Height: 6mm (.236 inches)
Angle: 40°
http://umstrans.com.au/belts/index.htm
http://www.vbelts4less.com/V-Belt-Specs_ep_45.html
