KLM has a variety of different 737's:
KL405/406 are to/from AMM, and are probably either the 800 or 900 series plane, since I believe it's out of range for the 300 or 400. It's hard to say if there are any "better" coach seats on KL's 737's. The 737-900 has a nominal "WBC" section (2-3 seating instead of 3-3 seating, whooppee
), but otherwise pretty much all 737 seats (even "Europe Select") are essentially the same. Note that the divider between WBC/EuropeSelect and regular coach can be moved and is not necessarily consistent with the seat maps linked above.
I assume AMM, like TLV (which I've done on KLM), lets you select your seats ahead (you can't select ahead for intra-Europe). If so, you should be able to tell from the seat map whether you're on the 800 or 900. NWA's version of the seatmap, however, may omit rows like the bulkhead and the exit rows.
Avoid "bulkheads," because they're not a real bulkhead, but just a curtain in your face with no extra legroom. And avoid rows without windows. You should be able to tell from the seatmaps linked above.
Exit rows have more legroom, but on KLM you cannot put anything on the floor (even under the seat in front of you) in the exit rows during takeoff and landing, which is a pain for long flights like AMM or TLV. Also, the exit row window seats have no armrests on the wall, and the seats may be shorter in the butt than normal seats (i.e., less of your leg is supported by the seat). Still, the legroom is nice.
I don't believe you can select exit rows ahead of time (especially if the reservation was through NWA). But if you use nwa.com/klm.com checkin, you can usually snag 'em.
I hope this helps!