There are 2 panels on the back, one on each side, that cover the openings for the lower 2 drawers. This image shows the back with one panel removed. You'll notice screw holes down each side, very close to the edge of the side boards of the desk. If you look closely, you'll see a nail hole in the center of the top and the bottom, right next to a screw hole.
Here is an image of the other side, with the panel still installed. Notice the screws with the finish washers. Both panels had the same screws and finish washers, in the same pattern, and both panels had a small nail in the center at the top and bottom of the panel.
OK, so here is what I'm curious about: Those screws are so close to the edge of the side boards of the desk that they had to be screwed in at angle in order to catch the board. Both sides were done the same way.
This image was taken through the front of the desk with back panel installed. Notice the hole that I circled. You can see light through the hole in the panel. That hole will eventually be filled with an angled screw.
I see 3 possible reasons for the panels to be sized so that the screws had to be angled:
1 - They were cut too small to begin with and the builder decided not to cut bigger ones.
2 - The panels were cut to the size that the builder wanted, attached only with the 2 nails and sometime later someone else decided to add screws and finish washers. They are certainly not "modern" screws, so if the attachment method was modified, it was done quite some time ago. Just FYI...the nails were not loose. After removing the screws, I had to pound the panel off from the inside.
3 - The screws were purposely put in at angle during the initial build so that they didn't split the side boards. (Seems to me that slightly larger panels and pre-drilling would have served the same purpose.)
Any other thoughts or reasons that you can think of?